Don't run full-data backups when backup is disabled

If the scheduled job fires but backup is disabled or the device is
not yet provisioned (i.e. has not yet finished going through setup),
bow out gracefully without running any backup operations.  Also, even
if a backup is directly invoked (e.g. via adb), verify again right
before we start collecting app data, and abandon the operation in
that path as well.

(This is redundant; having only the latter test would suffice, but
this lets us distinguish in the logging more easily.)

Finally, make sure that if we were waiting on setup before permitting
backup operations to begin, that we startup the full-data scheduling
as well as the [separate] key/value scheduling.

Bug 19197062

Change-Id: I3d8fb650c50f946d8ed7ac7170df361c707f2528