Make input dispatcher only ANR for foreground windows.

Redesigned the input dispatcher's ANR timeout mechanism so it is much
closer to Froyo's policy.  ANR is only ever signalled if the dispatcher
is waiting on a window to finish processing its previous event(s) and
there is new pending input.

In the old code, we tracked the dispatch timeout separately for each
input channel.  This was somewhat complicated and also resulted in the
situation where applications could ANR long after the user had pushed
them into the background.

Change-Id: I666ecada0952d4b95f1d67b9f733842b745c7f4b