transcoding: use __ANDROID_API_FUTURE__ in __builtin_available check

__builtin_available 31 fails because we're not officially 31 yet.
Use __ANDROID_API_FUTURE__ for now.

bug: 178144708
test: builds; transcoding unit tests; manually test transcoding works.
Change-Id: I4ccf05f3858feb9e22eb294b4443edc830737204