The SipService used to take an action string and broadcasts an intent with
that action string when an incoming call is received. The design is not safe
(as the intent may be sniffed) and inflexible (can only received by
BroadcastReceiver). Now we use PendingIntent to fix all these.
Companion CL: https://android-git.corp.google.com/g/#change,71800
Change-Id: Id12e5c1cf9321edafb171494932cd936eae10b6e
