Ensure audio mode is updated when the foreground call changes.

In the past, the audio mode (voip/modem) was triggered in a few cases
where new calls started or calls swap, as a side effect of notifying the
CallAudioRoute/ModeStateMachines of those operations.

There are two cases where that was not enough:
1) When an ongoing unholdable pstn call is disconnected due to a voip
call being answered.
2) When swapping between an ongoing PSTN call and VOIP call in some cases
the new foreground call would not correctly be set -- this caused the fg
call to be tracked incorrectly and the audio mode not to be set.

To correct this, updated the logic to determine which call is the
foreground call so that it will preferentially use an active or dialing
call which is not locally disconnecting (ie this is the original bug case)
to become the foreground call.  Failing that it'll fall back to the old
behavior of choosing the first call.

Finally, to make audio mode changing more reliable, added code which is
executed on change of foreground call.  We will now send a
FOREGROUND_VOIP_MODE_CHANGE event to the CallAudioModeStateMachine so that
it can properly transition between voip/modem audio modes.  This signal
does overlap with some of the other cases (new call added for example),
however the net effect is no audio mode state transition would happen in
those cases.

Test: Manual test for repro steps mentioned in bug.
Test: Added new unit test coverage for this case in CallAudioManagerTest
Fixes: 289861657
Change-Id: Ie337985f8dea9ca688af4f6a08a5a9dc9a92943b