android: fix build with android gradle plugin 7.2

[CXX1415] .../gstreamer/subprojects/gst-examples/playback/player/android/app/build/.cxx/Debug/5f6v5o23/arm64-v8a/android_gradle_build.json debug|arm64-v8a : gstreamer_android-debug-gst-build-arm64-v8a.abi 'gst-build-arm64-v8a' is invalid. Valid values are 'armeabi-v7a, arm64-v8a, x86, x86_64'

Move arch-specific directories into their own subdirectory.

It seems like somewhere in gradle is assuming the last path component is the abi of the built files when that may not actually be the case. Workaround this.

Fixes gstreamer#1284 (closed) Fixes gst-docs#103 (closed)