Added Lock for mCallbacks and test for Concurrency

mCallbacks in CastControllerImpl is now
thread-locked by a guard of the same name.

The test implemented in CastControllerImplTest fails
in previous builds (bug 79419738) and passes now.
In order to properly test,
CastControllerImpl::fireOnCastDevicesChanged is
now VisibleForTesting.

Change-Id: I4160938b2da1749a4370d902e314deaf445cda1a
Fixes: 79419738
Test: CastControllerImplTest::testConcurrencyOnMCallback