DeviceConfig: Split android:window_manager into android and window_manager

The keys are split according to whether they affect the system as a whole,
or simply the inner workings of the window manager.

Also encapsulates WindowManagerConstants properly and adds testing.

Bug: 139735771
Test: atest WindowManagerConstantsTest
Change-Id: I67c96a2bdd3140fe2ff9c6832d2d69f2e57115e2