Always reset screenshot preview to alpha=1

It's possible for the screenshot process to not get shut down as
soon as the UI disappears. If this occurs after a shared transition
(which sets the preview to alpha=0 to hide it as part of the
transition), the preview gets stuck at alpha=0 and isn't visible.
Always resetting the screenshot view and setting alpha=1 guarantees
that regardless of the state of the screenshot process, we always
start from a clean slate.

Bug: 242862442
Test: forced the screenshot process to stay up by never running
the finish callback in ScreenshotHelper, verified that we get the
invisible-preview behavior and that it's fixed with this change

Change-Id: I9b795928cba93f8cef19244105db2c7f31c0973b