Account for removal of perm group in Auto Revoke livedata

Ensure that the AutoRevokedPackagesLiveData does not crash if the
permission groups of one of the packages it watches becomes invalid

Fixes: 153820270
Test: Replace an app with a version of itself that requests fewer
permissions while the ManageStandardPermissionsFragemnt or AutoRevoke
fragment are open.

Change-Id: I3dce64bc227dd3c07630d58263586ebf5d9428a4