Closes https://github.com/MusicPlayerDaemon/MPD/issues/866

Sync with the Android NDK build scripts.

This flag is used by the Android NDK build scripts as well, and this
fixes a build failure (assembler error) with FFmpeg and NDK r21.

Using MPD from Android since quite some times now. I consider it very stable
now.

TODO: Not sure the app could be accepted on the play store without a valid
banner.

This seems to be required on recent Android versions (tested with Android 10).
This is also required for android TV services (cf. next commit).

This is done using Java reflection so that the project doesn't depend on
android compat libs.

Google Play refuses uploads with a lower targetSdkVersion.

This flag doesn't appear to work, and since we never had a problem
without the flag, implementing Google's recommendation is useless.

Don't export the symbols of all those static libraries.  Most
importantly, don't export the whole libc++ ABI.

These appear to be no longer necessary (tested with NDK r20b).

Closes https://github.com/android/ndk/issues/951

This implicitly defines __ANDROID_API__, which means we can drop the
"-D__ANDROID_API__=" parameter.

This is recommended on
https://android.googlesource.com/platform/ndk/+/ndk-release-r20/docs/BuildSystemMaintainers.md

It's not mentioned on
https://developer.android.com/ndk/guides/other_build_systems

Using this API function requires SDK level 23.

Fixes nullptr dereference when an exception gets thrown because there
is no ".eh_frame" section for unwinding.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/543