Fix answer ui with no SYSTEM_ALERT_WINDOW permission

If the user disallowed the "Display over other apps" permission then
incoming calls would no longer show up on the lock screen.

The problem was that if the permission was denied then we would attempt
to ask the user to give us the permission. This causes us to start an
activity while the in-call activity is starting which break things.

Fix was to never prompt the user to give us this permission. This is ok
because not having this functionlity doesn't break anything.

PiperOrigin-RevId: 159901233
Change-Id: I3c06ba327ef265fd7981931f49c44e00d1c25350