[sf] Update snapshots after screenshots

Screenshots can be taken for a partial hierarchy and may require
some of the layers to be temporarily reparented and their geometry
to be updated. Sceenshots use a copy of the layer snapshots and another
instance of the composition engine to render the contents to a
buffer.

There was an issue where the snapshots were updated before they were
copied and if the screenshot was taken during a period where there
were no visible region changes (and no reason to rebuild the snapshots)
we would pass invalid snapshot data when compositing to the display.

With legacy frontend, its harder to split the screenshot path and the
display composition path, so as a fix, update the snapshots after
screenshots are taken.

Bug: 282884552
Test: instrumented load to verify snapshots are updated correctly
Test: repro steps in bug

Change-Id: Ie778b02b57c5b1ddf0f09afe82675e04c5535ee5