Add permission to override display mode requests

This permission will be used for testing purposes
to override the display mode requests in DisplayModeDirector.

Bug: 159113268
Bug: 158316271
Test: m
Change-Id: Ibcafc50badcda929c3c3b446b6d0d9045c2a5b6d
Merged-In: I257636ae2a6ed74044e71e49a99024682c3a92c5