Fix bug where we could get stuck repeatedly launching an activity.

A previous change to avoid losing activities if their process
happens to be gone at the point of launch (by counting that
activity as having its state saved) has resulted in a problem
where an activity that crashes during launch will be repeatedly
relaunched.

This is fixed here by explicitly keeping track of our attempts
to launch the activity since it was last able to save its state,
and not keeping it around if it looks like the launch is
repeatedly failing.

Change-Id: Icefd952443b7eb1222f233db95e0157fc3dd72d1