libsync is an NDK/LLNDK library but it's missing "stubs" key. So, when
it is referenced by an APEX, it is bundled in APEX package.
By adding "stubs" property, we can make it a stubs library and APEXes
use it from the system instead of bundling it.
Note that the symbol(sync_wait) is exposed to APEX because it is used
by libui which is used by media APEXes again.
Bug: 158270824
Test: lunch mini_armv7a_neon # no VNDK
m com.andorid.media.swcodec
// see if libsync is not in the APEX
Change-Id: I39e682328acb5cc363a4242601e5bf1470938dac
78fc553d68