Make libsync a stubs library

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
Bug: 183656106
Test: lunch mini_armv7a_neon   # no VNDK
      m com.andorid.media.swcodec
      // see if libsync is not in the APEX
Merged-In: I39e682328acb5cc363a4242601e5bf1470938dac
Change-Id: I39e682328acb5cc363a4242601e5bf1470938dac