Enable dither for DismissView

It otherwise causes banding on devices with a small color palette.

Bug: 274938944
Test: Look for banding with maximum brightness and dark background
Test: Before https://screenshot.googleplex.com/8kHhVcps35MKqDj.png
Test: After https://screenshot.googleplex.com/Az5c9ykirCQpMXi.png
Change-Id: I62a10a8e9c7f2e8293b93f44d9f4d2fedfbc428a