Fixes the error:

 IllegalStateException: Not allowed to start service Intent { cmp=org.musicpd/.Main (has extras) }: app is in background

Needed for `requestLegacyExternalStorage` (commit ca02fb77).

This is a workaround for the new scoped storage design in Android 11:

 https://developer.android.com/about/versions/11/privacy/storage

This needs a proper solution eventually, but this quick fix will do
until we change "targetSdkVersion" to 30.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1061

The Google overlords require me to change to 29 or else I can't upload
new releases to Google Play.

 https://developer.android.com/distribute/best-practices/develop/target-sdk

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1059

Closes https://github.com/MusicPlayerDaemon/MPD/issues/891

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