This hides a particular symbol from the NDK stubs, while allowing the
symbol to be exposed to the VNDK through the LLNDK stubs. This doesn't
introduce any sort of versioning yet, this will need to change when we
add a new symbol to a new version of the VNDK.
Test: test_gen_stub_libs.py
Test: With my LL-NDK patches, inspecting the generated map files
Change-Id: Iee86aafda7985d6d7a016d0d5ff951505634913b