Always have a handler for PendingIntents sent in the system process

It is possible for deadlocks to occur when sending/processing
PendingIntents due to components like AMS holding global service
locks when dispatching the broadcast without specifying an
handler and the receiver calling back into AMS.

We now set a default handler to process the broadcast on if we
are in the system process and a handler wasn't specified.

Bug: 19502993
Change-Id: Iccde32c6c1df997784155bc41ef39e52ee0f7243