Implement new API of external display settings (1/2)

- Using DisplaySettings class for storing the display settings.
- Define flags in WindowManager.
- Have direct IWindowManager APIs to set and change display settings at
  runtime.
- Mark TODO to original usage of the flags.
- Add test case of DisplaySettings.
- Expose some APIs for CTS usage.

Bug: 114338689
Test: atest DisplayWindowSettingsTests
Test: atest CtsApacheHttpLegacy27ApiSignatureTestCases
Change-Id: I64ed14866d45cd5817fc3c895b6110c79c37b0ad