[Android] Put Java JNI function in a separate static library

[grendello]

In dotnet/android#9006 we are working on linking the .NET for Android
runtime dynamically at the application build time.

Linking includes all the relevant BCL native libraries, and one of the goals is to
hide all the exported symbols used as p/invokes by the managed BCL
libraries. This is because p/invoke calls are handled internally and,
with dynamic linking of the runtime, there is no longer any reason to
use dlopen and dlsym to look them up, they are all resolved
internally, at the link time.

Symbol hiding works fine thanks to the --exclude-libs clang flag,
which makes all the exported symbols in the indicated .a archives to
not be exported by the linker. However, System.Security.Cryptography.Native.Android
is special in the sense that it contains one symbol which must not be hidden,
Java_net_dot_android_crypto_DotnetProxyTrustManager_verifyRemoteCertificate.

The above function is a Java native method implementation, and it
requires that not only its name follows the Java JNI naming rules, but
that it is also available for the JVM to look up using dlsym.

I tried using the --export-dynamic-symbol clang flag to
export just this function, but it doesn't appear to work no matter
where I put the flag in relation to reference to the .a archives.

Instead, the problem can be dealt with by putting the JNI function in a
separate static library, so that I can link it without changing symbol
visibility, while making all the System.Security.Cryptography.Native.Android
symbols invisible.

[dotnet-policy-service]

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