Fix race condition generating READY and NOT_SUPPORTED statuses.

The race condition only affects when the client registers for several (all) location listeners.
And the side efects are benign: only the measurement and navigation message status are incurrectly
being sent to the application, but there are no crashes or any real data from GPS being
misscommunicated.
Also:
- cache the last reported status to filter sending notifications when no changes have occurred
- do some cleanup and refactoring in the code changed

Change-Id: I0692e6b70847dc1ee092d7a05a2c6ba3cd9fa147