Transcoder: Send more granular heartbeats on transcoder start up.

The transcoder watchdog timer starts when a request is sent to
the transcoder. But configuring and starting the transcoder may
delay the first heartbeat under high load. This CL mitigates this
in three ways:
- Send heartbeat during transcoder configuration.
- Send heartbeat when track produce output formats during start.
- Count samples written to the muxer from all track as progress.

Fixes: 183388518
Test: Transcoding unit tests.
Change-Id: I0fe935b33bc0601a0671e6ba39e1e3f528d1132b