Support Magnification capabilities Settings value

To have more flexibility and have consistency with legacy behavior,
Uses can choose single/multiple modes in settings.

To support it, we have the following behavior
1. Switching magnification mode is allowed only when the capabilities
is all.
2. If the capabilities doesn't support currnet magnification mode,
the magnification mode will be fell back.
3. Set window magnification connection to null if the magnification
settings is not turned on or the magnification capabilities is
full-screen.

Bug: 155965892
Test: atest WindowMagnificationManagerTest
Test: manual test: select one or two magnification modes to see
      the behaviour.
Change-Id: If32e2a038e64ed7a7bab9ec5a3d444b542d30161