Screensaver: Always dismiss on touch in preview mode.

This change adds an additional touch handler when in preview mode that
dismisses the dream on any touch.

Fixes: 416412668
Flag: com.android.systemui.dream_preview_tap_dismiss
Test: atest DismissTouchhandlerTest
Change-Id: I201ffead2d2cf58bf91b05bef69c777c1c89e2d9