Improve how we manage the previous app.

Setting it when a new activity is being resumed is too soon, because
things like an activity launching an exiting (without being seen by
the user) can knock out the real previous app that we want.

So now we set it when an activity is stopped.  At this point it is
going to move from the preceptible to background oom adj, so it is
a good point to determine whether it should be a previous app to
instead put it to that oom adj.  This also avoids things like
activities that start and immediately finish from impacting the
previous app.

Further, we keep track of the time each activity was last shown, and
use this to further filter what is set as the previous app.

Change-Id: I72d1cac4de0cc2d4598170296028f11b06918d4f