Don't clear visible-behind activity if it is top

Previously if an activity requested to keep running behind
translucent activities (Activity.requestVisibleBehind()) and then
converted itself to opaque (Activity.convertFromTranslucent()), we
would clear the visible-behind activity. This change tests to see
if the top activity is the visible-behind activity and does not
clear it in that case.

This change also clears the visible-behind activity whenever it
comes back to the front. That forces the activity to call
requestVisibleBehind() each time it is resumed.

Fixes bug 17648436.

Change-Id: Id0fc4d7e2a2b907675305d98bad1b08cb610919e