Add libstagefright_foundation to native_shared_libs of apex

com.android.media.swcodec apex uses libstagefright_foundation.so
which has an apex variant that excludes links to libbinder.so.
On devices that doesn't use apex, the apex is a folder and these
libs by default will become symlinks to the lib in /system/libs.
This doesn't work for swcodec because the system variant links
to libbinder which is inaccessible from apex's namespace.

bug: 175592420
test: build wembley-userdebug;
- manually check that the out /apex/com.android.media.swcodec/libs
has the real libstagefright_foundation.so, not symlink
- 'readelf' the lib and shouldn't need libbinder.

Change-Id: Ic2dca3eb3395693346531d62bd3b6574d8c85d5f