video: Fix crash during video recording

In pause-resume scenario, MPEG4Writer::start is
called to resume recording, which also resets
start time for writer to -1. If this happens
between two tracks updating the writer start
time, it can end up having the larger of the
start time of the two tracks, which results is
crash later due to assert.

Added change to set mStartTimestampUs to -1 only
when writer is being started, and not resuming from
pause.

CRs-Fixed: 692122
Change-Id: Ia98656acfa3912876d01e92b36c5e06a8c84eb7b