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

libsync/Android.bp

@@ -25,6 +25,12 @@ cc_library {
     recovery_available: true,
     native_bridge_supported: true,
     defaults: ["libsync_defaults"],
+    stubs: {
+        symbol_file: "libsync.map.txt",
+        versions: [
+            "26",
+        ],
+    },
 }
 
 llndk_library {

libsync/libsync.map.txt

@@ -19,7 +19,7 @@ LIBSYNC {
     sync_merge; # introduced=26
     sync_file_info; # introduced=26
     sync_file_info_free; # introduced=26
-    sync_wait; # llndk
+    sync_wait; # llndk apex
     sync_fence_info; # llndk
     sync_pt_info; # llndk
     sync_fence_info_free; # llndk