audio: don't apply ramp if track is paused before the first mix

If a track is paused right after start before it gets a chance to mix
the first frame. Don't apply volume ramp if it resumes later.

Test: Basic audio sanity.  Partner CL.
Bug: 123063164
Change-Id: I969ff1b6edce8faf77f2cdc481db9c1e3ef6d3dd
1 file changed