Removed unused variable macro usage

Note: On host toolchains, __unused doesnt seem to be present. Hence
updated all instances to use __attribute__((unused)) instead. Also
unused variables have been deprecated and code has been updated
to use br.skipBits() instead.

Test: Build libstagefright_foundation on Android
Bug: 151789258

Change-Id: Ia2af1467803f3325aa838c8d57809790d5ce19a0