SipService: add wake lock for incoming INVITE packets.

+ Keep the wake lock for 500ms. (Some measurements on N1 indicate 160~180ms
  needed to bring up InCallScreen but since INVITE doesn't come in frequently
  we can be more generous just to be safe.)
+ Move MyWakeupLock out of SipService so SipSessionGroup can use it without
  awkward inter-dependency with SipService.
  + Add acquire(int timeout) to be used to create the "timed" wake lock.

http://b/issue?id=3081828

Change-Id: Iffd1d78d1a5cae9f795252ada75310917095204d