Disallow interception of multitouch events

The shade mishandles multitouch events when the first touch
originates in the status bar, which results in the second touch
being an ACTION_DOWN instead of ACTION_POINTER_DOWN. This causes
multiple problems, so it's preferable to not intercept these
events when we detect them.

Fixes: 198553252
Test: manual and atest
Change-Id: Ib420e0aa207610d5493e7db904eaf31da221a2bf