Fix issue #6284404: ArrayIndexOutOfBoundsException in...

...FragmentManagerImpl.restoreAllState

This was a bug related to the difference between the pre- and post-HC
behavior of onSaveInstanceState().  Prior to HC, state was saved
before calling onPause().  Starting with HC, it is saved between
onPause() and onStop().  To maintain compatibility with existing
applications, there is a check in ActivityThread for pre-HC to in
that case emulate the behavior of old applications, still calling
onSaveInstanceState() before onPause() but using the state later.

One of the special cases we had to deal with in the old model of
saving state before pausing was restarting an activity that is
already paused.

Consider, for example: you have two activities on screen, the one on
top not fullscreen so you can see the one behind.  The top activity
is resumed, the behind activity is paused.  In the pre-HC world, the
behind activity would have already had its state saved.

Now you rotate the screen, and we need to restart the activities.
We need to destroy the behind activity and create a new instance,
but the new instance has to end up in the paused state.  To
accompish this, we restart it with a flag saying that it should
end up paused.  For the pre-HC world, since it ends up paused,
we need to make sure we still have its instance state kept around
in case we need it because we can't regenerate it (since it is
already paused).

So that is what the changed code here is doing.  It goes through
the normal create/start/resume steps, but holds on to the current
saved state so that it isn't lost when resume clears it, and then
puts the activity back to paused and stuffs that old saved state
back in to it.

The problem is that this code was doing it for every application,
even HC apps.  So we end up in a bad state, when a HC app has its
saved state sitting there as if it had been saved, even though it
is only paused.  Now if we go to restart the activity again, instead
of asking it for a new saved state (as we should for a HC app as
part of stopping it), we just re-use the existing saved state again.

Now this wouldn't generally be a huge problem.  Worst case, when we
restart the activity yet again we are just instantiating it from
the same saved state as we used last time, dropping whatever changes
may have happened in-between.  Who cares?  All it has been doing is
sitting there in the background, visible to the user, but not something
they can interact with.  If the activity made changes to its
fragments, those changes will be lost, and we will restore it from
the older state.

However...  if one of those fragements is a retained fragment, this
will *not* appear in the saved state, but actually be retained across
each activity instance.  And now we have a problem: if the retained
fragments are changed during this time, the next activity instance
will be created from the most recent state for the retained fragments,
but the older state for everyting else.  If these are inconsistent...
wham, dead app.

To fix this, just don't keep the saved state for HC apps.

Also includes a small optimization to ActivityStack to not push
the home screen to the front redundantly.

Change-Id: Ic3900b12940de25cdd7c5fb9a2a28fb1f4c6cd1a