Send broadcasts as Shell.

The BUGREPORT_STARTED was being sent as root, which would cause it to
bypass Shell restrictions when DISALLOW_DEBUGGING_FEATURES is set.

Also added the missing --receiver-background and unhardcoded shell's uid
from previous changes.

BUG: 26814173

Change-Id: I867361efafb96c7e709f83806a9301afc637a27e