Fix GpsLocationProvider wake lock book keeping.

The GpsLocationProvider typically acquires a wake lock
before sending a message to its handler then releases it
after the message has been handled.

There were two cases where messages might be removed from
the handler, resulting in the wake lock being released.

There were also two cases where background tasks were being
started while not holding a wake lock for the duration.

Fixed these issues and marked the GpsLocationProvider handler
as asynchronous too so that it doesn't accidentally get
blocked by traversals if it happens to share a thread with some UI.

Bug: 7057752
Change-Id: I8e12fc91ae943e84db068c08ec809879537503c6