Consume panel touch event to prevent incorrect processing

Touches into NotificationPanelViewController should always come
directly from NotificationShadeWindowViewController. Under certain
configurations, they are being routed directly to the view for
processing. In this case, mark the touch as handled to prevent
any downstream effects.

This fixes swipe down underneath notifications for certain
devices.

Fixes: 433897926
Test: manual - AOSP builds, retest all swipe interactions
Flag: EXEMPT bugfix
Change-Id: I897367874b52712b10569278eed8db9f678047e2