avsync: add new API to reset sync mode

This is for a work around in westeros-sink where the avsync mode is
initialized as VMASTER, but a short time later, it will update to
AMASTER.

Westeros-sink needs to show frames in PAUSED state, but clock source is
not always available at that time.

Change-Id: I205f8d47f6301f162f9fb7a86e0bfef497681780
2 files changed