Possible fix to issue #3213749: NPE at...

...android.app.Fragment.startActivityForResult(Fragment.java)

Make sure to remove all pending messages when AbsListView is detached
from its window.

But...  that's not enough.

It turns out that when a fragment's views are animating away, they of
course don't get detached until after the animation is done.  However
the fragment itself is immediately destroyed, leaving its live views
still going after that.

Here's a possible solution: when fragment manager initiates an animation
on a fragment whose views are being removed, it makes note of that so
it can hold off on destroying the fragment until the animation is over.

There are a lot of interesting race conditions here, if further operations
happen on the fragment while it is being animated.  I think the code here
does something sensible, and it does seem to work for the situations I
have tested, but it is hard to know all of the edge cases that may happen.

Change-Id: I4490ce8862a9bb714c7ea54baca3072c62126388