Transcoder: Preserve source track's bitrate by default.

When no bitrate is passed in the transcoder now preserves the
source track's bitrate. If the bitrate is not available as metadata
the transcoder will sample the source track and estimate the bitrate.

Fixes: 160648740
Test: Unit tests.
Change-Id: I4e0284128aaa1116f5b61d908b52582b2ed3d7e4