contexthub: Handle service death

Register for notification of service death for callbacks registered with
the context hub HAL. Upon this notification, unregister the callback to
avoid crashing if an event occurs prior to the service coming back up.

Bug: 36202367
Test: adb shell stop, confirm death notification received via log;
  vts-tradefed run commandAndExit vts --module VtsHalContexthubV1_0Target
Change-Id: I3c25229806cb9f2a116007939c752841edbf0985