Clean up audio mode setting in telecomm.

Audio mode was being set too often by triggers which could come in
outside of a call: headset plugged in, bluetooth plugged in.
Do a check for focus before allowing these triggers to take effect.

Additionally, audio settings were updated for any added call, even
those which may not need audio yet (NEW calls which have not
transitioned to DIALING or RINGING...or anything else). Don't do that.

Bug: 15589644
Bug: 16132361
Change-Id: Id8425a15cd89233159323387b1e86f432c09016a