Transcoder: Don't fail test if codec changes configured bitrate.

Change VideoTrackTranscoderTests#PreserveBitrate to check the
bitrate passed in to the encoder rather than what comes out of the encoder.
Recent minimum quality work allows the encoder to "shape" the
MediaFormat if the quality is too low. This test triggered that.

Test: Transcoder unit test
Fixes: 184920307
Change-Id: Ia5edfc86ff1dff28d591cb5c011f26f9febd7a1d