Update PackageMonitor to recognize new 'replacing' semantics

The ACTION_EXTERNAL_APPLICATIONS_UNAVAILABLE broadcast now uses the
EXTRA_REPLACING intent extra when it is sent as part of an upgrade operation
on a forward-locked application.  Update PackageMonitor to recognize this
new information and express it appropriately to the observer.

Bug 11988313

Cherry-pick from master.

Change-Id: Iecea1876ffc918f23f9fa5845f1f89ed8d740dd5