Synchronize updates of missed call counts and add more logging.

When the missed call count stored in MissedCallNotifierImpl was updated,
there was a rare chance for the missed call count to be cleared while it
was being updated.  mMissedCallCounts is a concurrent hash map and the
references it stores are AtomicIntegers; getting an element and then
updating it can't be guaranteed to be thread safe.  Also, in the case of
updating the missed call count we'd update the value and then re-fetch it
when sending the intent to dialer; this means it could have been cleared
in the process of being updated.

Test: Ran MissedCallNotifierImpl tests.
Test: Performed missed call regression testing to verify that missed call
notifications and clearing takes place in Dialer as expected.
Test: Added unit tests for dialer-handled set/clear of missed call count.
This doesn't test concurrency per se, but it helps improve test coverage.
Fixes: 168695531

Change-Id: I76389500bf8d91f029123178060db6ff38e398eb