Add support for JNI libraries to android_app modules

Make android_app modules a MultiTargets module, which means the
common variant will have a list of Targets that it needs to handle.
Collect JNI libraries for each Target, and package them into or
alongside the APK.

Bug: 80095087
Test: app_test.go
Change-Id: Iabd3921e1d4c4b4cfcc7e131a0b0d9ab83b0ebbb