audio: support bitdepth meta in webm writer

bitdepth meta is not carried when assembling a new webm from orginal
clip with media muxer, which breaks native decoder CTS cases.

To fix this, we need to reserve bitdepth info when converting from
AMessage to MetaData, and insert bps in webm writer.

Change-Id: I97ca473be26ffc32df75b6b78948b802081b234e
CRs-Fixed: 1053234