Update Android Circle CI docker image to NDK 18

[tobrun]

This image brings: NDK 18 + build tools 28.0.3

[tobrun]

On android-debug-arm-v7 and android-release, we are hitting:

In file included from /src/src/mbgl/actor/mailbox.cpp:1:
In file included from ../../../../../../../include/mbgl/actor/mailbox.hpp:3:
In file included from ../../../../../../../include/mbgl/util/optional.hpp:3:
/android/sdk/ndk-bundle/sources/cxx-stl/llvm-libc++/include/experimental/optional:11:2: error: "<experimental/optional> has been removed. Use <optional> instead."
#error "<experimental/optional> has been removed. Use <optional> instead."
 ^
In file included from /src/src/mbgl/actor/mailbox.cpp:1:
In file included from ../../../../../../../include/mbgl/actor/mailbox.hpp:3:
../../../../../../../include/mbgl/util/optional.hpp:8:18: error: use of undeclared identifier 'std'
using optional = std::experimental::optional<T>;
                 ^
../../../../../../../include/mbgl/util/optional.hpp:10:19: error: use of undeclared identifier 'std'
using nullopt_t = std::experimental::nullopt_t;
                  ^
../../../../../../../include/mbgl/util/optional.hpp:11:11: error: unknown type name 'nullopt_t'
constexpr nullopt_t nullopt = std::experimental::nullopt;
          ^
../../../../../../../include/mbgl/util/optional.hpp:11:31: error: use of undeclared identifier 'std'
constexpr nullopt_t nullopt = std::experimental::nullopt;
                              ^
In file included from /src/src/mbgl/actor/mailbox.cpp:1:
../../../../../../../include/mbgl/actor/mailbox.hpp:39:5: error: no template named 'optional'
    optional<Scheduler*> scheduler;
    ^
/src/src/mbgl/actor/mailbox.cpp:31:32: error: no matching member function for call to 'shared_from_this'
        (*scheduler)->schedule(shared_from_this());
                               ^~~~~~~~~~~~~~~~
/android/sdk/ndk-bundle/sources/cxx-stl/llvm-libc++/include/memory:5340:21: note: candidate function not viable: no known conversion from 'mbgl::Mailbox' to 'std::__ndk1::enable_shared_from_this<mbgl::Mailbox>' for object argument
    shared_ptr<_Tp> shared_from_this()
                    ^
/android/sdk/ndk-bundle/sources/cxx-stl/llvm-libc++/include/memory:5343:27: note: candidate function not viable: no known conversion from 'mbgl::Mailbox' to 'const std::__ndk1::enable_shared_from_this<mbgl::Mailbox>' for object argument
    shared_ptr<_Tp const> shared_from_this() const
                          ^
/src/src/mbgl/actor/mailbox.cpp:61:32: error: no matching member function for call to 'shared_from_this'
        (*scheduler)->schedule(shared_from_this());
                               ^~~~~~~~~~~~~~~~
/android/sdk/ndk-bundle/sources/cxx-stl/llvm-libc++/include/memory:5340:21: note: candidate function not viable: no known conversion from 'mbgl::Mailbox' to 'std::__ndk1::enable_shared_from_this<mbgl::Mailbox>' for object argument
    shared_ptr<_Tp> shared_from_this()
                    ^
/android/sdk/ndk-bundle/sources/cxx-stl/llvm-libc++/include/memory:5343:27: note: candidate function not viable: no known conversion from 'mbgl::Mailbox' to 'const std::__ndk1::enable_shared_from_this<mbgl::Mailbox>' for object argument
    shared_ptr<_Tp const> shared_from_this() const
                          ^
/src/src/mbgl/actor/mailbox.cpp:88:32: error: no matching member function for call to 'shared_from_this'
        (*scheduler)->schedule(shared_from_this());
                               ^~~~~~~~~~~~~~~~
/android/sdk/ndk-bundle/sources/cxx-stl/llvm-libc++/include/memory:5340:21: note: candidate function not viable: no known conversion from 'mbgl::Mailbox' to 'std::__ndk1::enable_shared_from_this<mbgl::Mailbox>' for object argument
    shared_ptr<_Tp> shared_from_this()
                    ^
/android/sdk/ndk-bundle/sources/cxx-stl/llvm-libc++/include/memory:5343:27: note: candidate function not viable: no known conversion from 'mbgl::Mailbox' to 'const std::__ndk1::enable_shared_from_this<mbgl::Mailbox>' for object argument
    shared_ptr<_Tp const> shared_from_this() const
                          ^
In file included from /src/src/mbgl/actor/mailbox.cpp:1:
../../../../../../../include/mbgl/actor/mailbox.hpp:39:26: error: private field 'scheduler' is not used [-Werror,-Wunused-private-field]
    optional<Scheduler*> scheduler;
                         ^
10 errors generated.

Which are patchable, but android-gnustl-arm-v7 isn't as NDK 18 removes support for gnustl:

Release notes:

• GCC has been removed.
• LLD is now available for testing. AOSP is in the process of switching to using LLD by default and the
  NDK will follow (timeline unknown). Test LLD in your app by passing -fuse-ld=lld when linking.
• gnustl, gabi++, and stlport have been removed.
• Support for ICS (android-14 and android-15) has been removed. Apps using executables no longer need to provide both a PIE and non-PIE executable.
• The Play Store will require 64-bit support when uploading an APK beginning in August 2019. Start porting now to avoid surprises when the time comes. For more information, see this blog post.

@kkaefer:
Thoughts on this?
Should we hold off on upgrading for the time being?
Start maintaining two different images?

[stale]

This pull request has been automatically detected as stale because it has not had recent activity and will be archived. Thank you for your contributions.

[stale]

This pull request has been automatically detected as stale because it has not had recent activity and will be archived. Thank you for your contributions.

[tobrun]

This landed