prevent concurrency issues during LocationManager init

systemReady() was returning before the LocationManagerService was
actually ready. Applications making LocationManager transactions
during their startup could possibly hit a race condition with the
yet-uninitialised LocationManagerService.

To guarantee that LocationManagerService is actually ready before
returning from systemReady(), we simply do the startup work on the
thread that called systemReady(), rather than spin up a secondary
thread to do the work asynchronously.

LocationWorkerHandler still needs a thread to do its work on, so
rather than have it run on the secondary thread that was
previously used for systemReady()'s work, we create a HandlerThread
for it.

Additionally, LocationManagerService.init() really needed to grab
lock for some of the things it was doing. I moved all of the code
that could benefit from mutex protection to a single section of
systemReady() and wrapped it up with a lock while I was at it.

Bug: 7723944
Change-Id: I51d480e2781622c3a14769c3a2019a2407dcfd8a