Assorted cleanup and refactoring

Will minimize future changes:
-Never return null GpsStatus to avoid breaking legacy clients
-Allow null location callback complete callbacks
-Return void from sendExtraCommand
-Combine appops and permissions checks into a single helper
-Allow passive provider to remember last locations even when there is
no active passive request.
-Add a couple new injectable helpers to support future changes along
with fakes for testing.

Test: presubmits + manual
Change-Id: If22bee787c5ab5c8c09d20e4742a8b5d1b7f0b59