audioflinger: add watchdog on main mutex

Check every 10 seconds if main audioflinger mutex can be acquired and
trigger audioserver process restart if not.
This will allow to capture tombstones of locked up situations and avoid
the disruption of a system_server restart.

Bug: 30737845
Bug: 30388410
Change-Id: I8a78b8cf813982f70ea598a6d42affc0ecaa76c9