Reset screenshot view correctly at animation start

During the dismissal animation, we fade out the screenshot view and
translate it off screen. However, we never reset these values, so for
any successive screenshots during the same screenshot process
instantiation the screenshot view is effectively invisible (since it's
both alpha=0 and translated offscreen).

Normally, this is irrelevant since the screenshot process is destroyed
once dismissal is complete (and restarted, with correctly reset
alpha/translationX properties, for any later screenshots). However due
to an unrelated bug (see ag/29549611) it's possible to enter a state
where the screenshot process never gets destroyed, and these parameters
are never reset.

This change resets the parameters so that even if the screenshot process
remains up, the animation is correctly visible.

Bug: 370722626
Fix: 370722626
Test: manual -- take a screenshot, then take another screenshot using a
different invocation method while the UI is still up, wait for
dismissal, and verify that further screenshots are correctly visible
Flag: EXEMPT trivial fix

Change-Id: Ia92afc43919ddda8d8329714e332456943aebe97