Clear touch region on dream overlay container on start.

Since the updated TouchInsetManager logic only sets the touch regions
for surface controls of tracked views, the dream overlay's touch
region will not be set if no view is tracked. This changelist ensures
that the touch region for the overlay is cleared on attach.

Test: atest DreamOverlayContainerViewControllerTest#testRootSurfaceControlInsetSetOnAttach
Fixes: 276023607
Change-Id: I8cc96ba3c1d35510620ca6ed605e386fe11aecc1