SDK / exoplayer

The term "passthrough" was heavily overloaded. For clarity, split most
of its usage to different terms:
 * codec "bypass": no MediaCodec is used
 * "direct playback": no decoding occurs (but decryption may or may not)
 * "decrypt only codec": a MediaCodec used only to decrypt, not decode
 * "offload": playback to an offload AudioTrack.
 * "passthrough" is now only used in the sense of playing encoded audio
 * to a non offload AudioTrack.

PiperOrigin-RevId: 324984612