Add a flag to fix rotation to user rotation only.

It also does a few more things:
1. Add a command to manually test the flag.
2. Refactor DisplayRotation a bit to make it unit testable.
3. Add some core unit tests for DisplayRotation.

Bug: 113252523
Test: Manual test.
atest WmTests:DisplayRotationTests

Change-Id: I00c4a44e4d2f637590e8b13f7e6194624c04c58f