Mark screenshot preview as clickable

Currently, talkback initially announces the screenshot preview as
"Edit screenshot; image", but once the preview is tapped says
"Edit screenshot; button". The screenshot preview is clickable and
assigning it "android:clickable=true" fixes this issue (to make it
always say "Edit screenshot; button").

Bug: 215819076
Fix: 215819076
Test: manual, with talkback on
Change-Id: Ib184322fb7fdfe04a921f36fde14d4ceaea3a140