Intercept touch events for launcher preview root view

This change prevents any accidental touches on any widgets in the
preview.

Test: Tapping on launcher preview in wallpaper app should never activate
a widget.
Fix: 191623924

Change-Id: Iac7eab057ec33ff491365a993af75a0b06a27f7e
Merged-In: I98d462eca699cf368dcd5894f15584f280932ccc