Adds a new flag to toggle OverlayDisplayWindow interactability

Currently, OverlayDisplayWindow is treated as a system overlay and will
block any touch going through it. In default display, this will obstruct
testing process as apps will be behind the overlay.

The new flag is an extension of an existing DISABLE_MOVE_AND_RESIZE
constant in OverlayDisplayWindow.

Doc: go/connected-displays-overlay-display-testing

Bug: 402671705
Test: Manual test + presubmit
Flag: EXEMPT adding a new dev-only flag
Change-Id: I5e9ae96258088b9b9cd8f5de393d4c0a7d94424c