Process power state syncs asynchronously

The instance locking introduced in
cc5388bbe81aba635fb757e202d860aeed98f3e8 keeps the power state sane
between stop and the periodic task power sync.  However, locking on
an instance in the periodic task thread can potentially lock
that thread for a long time.

Fix this condition by executing power syncs in a background GreenPool.
Limit the syncs to 1 at a time per instance.

Closes-Bug: 1363231

Change-Id: Idbc00ddfcb7a6816433863ed1686ad8f7477d99b
Merge "Process power state syncs asynchronously"