transcoder: wait for actual encoder output format to start writer

bug: 154734285
bug: 158240484

test: unit tests. In particular:

run MediaTranscoderTests and MediaTranscodingServiceRealTests, and
check the output files. Should play with proper aspect ratio on
host and device.

Change-Id: I8254fd8bd42678a414cab5fb0f61c4ba56d6c114