Increases the falsing threshold when for the keyguard affordances

The minimum amount has been increased by 20dp and in addition the
Motion is cancelled whenever a POINTER_DOWN occurs, which should
help a lot with falsing.

Bug: 18298401
Change-Id: I04b1f30fd2555c08209224f4e84c610ecd7a33f1
(cherry picked from commit 1dc40650)