Re-enable vold watchdog.

Awhile back we moved vold over to use Binder, which means we can now
execute multiple parallel calls down to vold.  We instead have a lock
on the vold side to ensure certain calls continue to be serialized,
but all long-running operations now use callbacks.

Thus the vold lock should be low-contention, and any operations that
wedge vold should cause the watchdog to fire and help the device
recover.

Bug: 80472979
Test: manual
Change-Id: Ie2ec68f1fa0ff895153438fcebbe1f2a4e8be0a7