Support to add JNI of both archs in multilib build.

Use "LOCAL_MULTILIB := both" to install jni libraries of both archs in
multilib build.
The build system will package jni of both archs to the apk, or install
them to the right location on the system image and create symlinks,
extract .so files from prebuilt apk, etc if appropriate.

Bug: 15849902
Change-Id: I7e147b5a47db476584c38250de7b36c75ea40d81