Update git submodules

* Update external/libcxx from branch 'ndk'
  to 5dcd3bb23cac63b76acd24bd26f5b9c0dd31e03a
  - Don't strip libc++_shared.so.
    
    This will get stripped by gradle before it is included in the user's
    APK. Having a non-stripped version in the NDK allows user to debug
    libc++ issues, should they be so unfortunate.
    
    Test: ndk/checkbuild.py libc++ && file path/to/libc++_shared.so
    Test: Use this NDK in Studio for a CMake project using c++_shared,
        check that the APK has a stripped libc++_shared.so.
    Bug: android/ndk#749
    
    Change-Id: Ifd6972d84c3eceba8dd14f0360033b0516445299
    

* Update ndk from branch 'master'
  to dd0f579a20d0eac7900bd8bdbe0d875d61dcbf3e
  - Merge changes from topic "libcxx-debug"
    
    * changes:
      Mention libc++ not being stripped in the changelog.
      Add APP_STRIP_MODE and LOCAL_STRIP_MODE.
    
  - Mention libc++ not being stripped in the changelog.
    
    Test: None
    Bug: android/ndk#749
    Change-Id: Id8440efbdadacc43f566577d7cafc0e61cec2595
    
  - Add APP_STRIP_MODE and LOCAL_STRIP_MODE.
    
    These arguments allow the user to specify how their binaries should
    be stripped. APP_STRIP_MODE indicated what to do if LOCAL_STRIP_MODE
    is not set. LOCAL_STRIP_MODE always overrides APP_STRIP_MODE.
    
    The value of this argument is passed directly to the strip command, so
    valid values include anything accepted by strip, but commonly will be
    --strip-debug or --strip-unneeded.
    
    A special value may be used to indicated that the binary should not
    be stripped at all: none. This is helpful for users that want to
    preserve even debug info, perhaps during debugging or to delegate the
    stripping behavior to their packaging system (Studio does this).
    
    Test: ./run_tests.py --filter strip*
    Bug: android/ndk#749
    Change-Id: Ia0748cb7b9b92bcbc164f5156a6ad0e523314be2