Have isRinging return true when simulated ringing

UI components watch for the return value of isRinging in order to figure
out whether to silence a call on vol-down, so make isRinging return true
when simulated-ringing so that button still works.

Bug: 150320750
Test: atest BackgroundCallAudioTest
Test: manual
Change-Id: I300a789d674e4aa777ac2031f4f7286b4d595614
Merged-In: I300a789d674e4aa777ac2031f4f7286b4d595614