Add a dismiss button to screenshot corner flow

Instead of dismissing on TOUCH_OUTSIDE, add an explicit dismiss
button to the screenshot preview, and allow all other touches
(not on the actions shade or screenshot preview) to pass through.
Also change the timeout to 6 seconds.

Test: manual
Bug: 149097928

Change-Id: Ie63a216381c68129e9bedb69b7ba84f53923d027