Rework LocaleTracker to use Asynchronous CellInfo

SST's CellInfo support is now asynchronous, so the
LocaleTracker no longer should operate as a separate
threaded entity. Instead, rework it so that it is a
simple handler, and remove the synchronization that
was used to support threading.

-Remove thread-centric stuff - synchronized and
 thread/nonthread duplicate functions.
-Remove duplicate calls that would start tracking
 and instead track the state of the tracker with a bool.
-Allow LT to use unsolicited CellInfo to back off
 from its timer logic and reduce calls to the modem.

Bug: 37100068
Bug: 112608851
Test: atest FrameworksTelephonyTests
Change-Id: Ib1d7b5d8406bf6486d301fd7be87b329b2582b35