Fix screenshot preview border in RTL

The constraints were pulling the border "in" from the preview edge
instead of extending "out" around it, in RTL. This change makes it
show up correctly in both LTR/RTL layouts.

Bug: 190629016
Fix: 190629016
Test: visual verification
Change-Id: I89e88479f3a268a73da444c55b54db6cfe8f2781