Dismiss blocking disabled notification after touch.

Before, it would linger around, even after the user had touched it
and visited the settings screen. Feels like more standard behavior
for it to be dismissed at this point (user has clearly seen it,
and it doesn't really contain any info which needs to persist).

Bug: 25377379
Change-Id: I5608a3d735fd084f4493c0fcd51d9dcedf04dc6f