Fix touch intercept bug in ResolverDrawerLayout

ResolverDrawerLayout could get into a state where it thought a drag
was in progress, but since there was no distance to drag it would
return false from onTouchEvent for ACTION_DOWN. This would cause it to
intercept all future touch events, blocking interaction with
ResolverDrawerLayout child views. This would only happen if none of
the child views wanted the initial ACTION_DOWN event and it ended up
being processed in the ResolverDrawerLayout itself, such as in
response to touching in empty space or on the title text, and if all
available options fit in view without needing to expand the drawer.

Fix this by only setting the dragging bit if we're going to claim the
rest of the touch events after ACTION_DOWN.

Bug 18563698

Change-Id: Iab2ec38528697c9ff36bbb08b1f1210a6879d5ef