Passthrough mode can use a codec or not, but
the code only mentioned "passthrough" in most cases,
making the specific mode confusing.
For example both `MediaCodecRenderer` and
it's derived class `MediaCodecAudioRenderer`
had a private `passthroughEnabled` field,
but they were used for the opposite modes!
This change renames all relevant variables/functions
to explicit `CodecPassthrough` or `Bypass`.
PiperOrigin-RevId: 319225235
