Layer caption container surface behind corner input region.

Changes layering of caption to allow input in regions occupied by
caption view. This change combined with ag/23604820 will allow for mouse input in the gap left by rounded corners.

Bug: 286461778
Test: Manual; check mouse input near caption corners in both desktop
mode and legacy caption.

Change-Id: I49d5c105b52920a3b0aead04dbc87231e73bdfca