Create default InCallService with Context.BIND_IMPORTANT

Docs suggest that this is good to do when starting a service from an 
activity. 

Performance is too affected by b/18338152 to get accurate measurements
right now, but this change doesn't seem to hurt (~50ms faster across 
9 runs, 3 runs per reboot).

Bug: 18373617

Change-Id: I235120f23ab972157653c9863f99511adbac747b