Make overlay display valid in accessibility

Overlay display is used for multi-display development and testing.
Developers can interact with the content using a mouse.

This makes accessibility handle overlay display as a valid display.

Flag: EXEMPT behavioural tweak that only affects multi-display developers
Test: adb shell settings put global overlay_display_devices 640x480/320 \
      adb shell uiautomator dump --windows \
      and see overlay contents are included.
Bug: 400872190
Change-Id: If00e935e7c9a5662a5d94ae23391f7308fef70c8