Android: Fix header inclusion and linkage of libsync
Recent changes to the way libsync is included have not been propagated to the (deprecated) AOSP in-tree build-system. These commits patch it up in a way that is hopefully sensitive. Transitive header exports don't seem to be used at all currently and I don't know if LOCAL_EXPORT_SHARED_LIBRARY_HEADERS is available in the minimum Android version these files still "aim to support".