Don't queue multiple pending backups for the same app

Repeated install/replace of an app before it ran its first backup pass
would wind up enqueueing multiple backup requests, all of which would
run back to back when the time came.  This no longer happens.

Also, if a backup request is queued for an app that is then uninstalled
before that request is honored, we no longer fail in expensive and log-
intensive ways; we now fail cleanly, early.

Bug 3133212

Change-Id: I745f5b2f966a1c874f34a0107a438ad42fa7f005