Transcoder: Detect input/output frame count mismatch due to codec error

When setting operating rate for 4K videos the encoder will
silently drop frames but produce an EOS.
This commits adds a check after encoder EOS that logs a
warning if the number of output frames from the encoder is different
from the number of input frames to the decoder and throws an error
if no output frames are produced.

Bug: 175406816
Test: 4K unit test from ag/13237959
Change-Id: Ib58e26296160e9e4748d7562dca1034372e440ea