Optimize bg check flow.

No longer need to look up the application info, target SDK is
explicitly passed in to the check.  For the external method, we
change this to just checked to see if background is completely
disabled, which doesn't need a target SDK check (and is the only
thing any of the current clients care about).

Now allow SystemUI to put targets of notification pending intents
on the temporary whitelist when they fire, so developers can avoid
dealing with background restrictions in this case (if the user
interacts with their notification, they will temporarily be
considered in the foreground).

Remove any thoughts of enforing restrictions on registerReceiver(),
so we don't need to deal with target SDK versions there (which can't
be done all that efficiently).

Also bring back the old "allow starts coming from foreground apps"
only for the MODE_IGNORE app op, since it should provide some
better compatibility.

Test: ran them.

Change-Id: Id4ea7f992d12ce4bd8e54f1dbaeb4a460a3dee59