Fix caching flickering for unsecure displays

Rendering a cached set needs to respect the secure mode for the display.
Otherwise, compositing a secure layer onto an unsecure display would
incorrectly leak information in a cached set.

Bug: 193103018
Test: Chrome in incognito mode and verify that both screenrecord and
simulating a virtual insecure display does not leak Chrome's content.

Change-Id: I6d43e1c7de41f113477d9928b91ee0b286249751