SurfaceFlinger: add a backdoor to override DisplayManager policy

Added a new backdoor, for debugging purposes, to override DisplayManager
policy and allow SurfaceFlinger to freely switch refresh rates.

To override DisplayManager policy:
	adb shell service call SurfaceFlinger 1036 i32 1
To undo the override
	adb shell service call SurfaceFlinger 1036 i32 0

Bug: 152411921
Test: override policy by backdoor and observe dumpsys SurfaceFlinger
Change-Id: I14bb4722986f6df9100959eea66e5e3a3fa6f953