Extract TestDisplayWindowSettingsProvider class

This is useful for injecting TestStorage instead of the real disk in
hermetic tests. It should be used in more tests than just the
DisplayWindowSettingsProvider unit tests.

Flag: EXEMPT mechanical refactor
Test: atest WmTests
Bug: 362981229
Change-Id: I1a7ec7d236cad3c6b583ac3ad16a75ac2a5bc0d1