Drop all dispatcher state when dispatcher is disabled.

This patch makes the dispatcher drop all of its state when it is
disabled (when the screen turns off).  This ensures that the dispatcher
does not get stuck thinking a pointer is still down if the screen
turned off while the user was touching the display (such as a fat touch
while hitting the power button).

Bug: 3098344
Change-Id: If50ef5804870aa1acd3179fd4b40e3cda58dd39d