Refactoring to make adding a secondary easier

This commit contains refactorings intended to make the addition of a
secondary LocationTimeZoneProvider easier and clearer.

This change contains various refactorings to code and comments:
1) Start referring to the provider as the primary in some places.

2) Move the initialization timeout tracking to the
LocationTimeZoneProvider so each provider can be responsible for
tracking its own initialization timeout, giving the controller less to
do.

3) The PROVIDER_STATE_ENABLED has been split into _INITIALIZING,
_CERTAIN and _UNCERTAIN variants which are determined by the most recent
event the provider has sent. The controller therefore doesn't need
to track this either.

4) Introduction of a "single runnable" for the uncertainty timeout
managed by the ControllerImpl.

5) Many ControllerImplTest changes resulting from all the above and to
maximize tests readability.

Bug: 152744911
Bug: 149014708
Test: atest services/tests/servicestests/src/com/android/server/location/timezone/
Change-Id: I64b9cc33c5f97f2edb1063a8133434bfd8e9dda1