transcoding: use builtin_available macro instead of __ANDROID_APEX__

This is a follow-up of Id0c23d90b69de3cd5ddfd73de6ebcf7154cb2b1d.
The uses of the Android S APIs are correctly with __builtin_available
guard.

Bug: 177365934
Test: m
Change-Id: Ib70e7c5128449c3e1abe1df79ca20e22bd7af781
3 files changed