fix(magnification): make always on toggle unavailable when capabilities is window only

As b/328787031, we make the MagnificationAlwaysOnPreferenceController observe the magnification capabilities then update the preference enabled state. Therefore, when changing the capabilities to window mode only, the preference will become unavailable.

We use the bug-fix flag to verify the fix with rollout process.

Bug: 328787031
Flag: ACONFIG com.android.settings.accessibility.hide_magnification_always_on_toggle_when_window_mode_only DEVELOPMENT
Test: manually flip the flag
      atest MagnificationCapabilitiesTest
      atest ToggleScreenMagnificationPreferenceFragmentTest
      atest MagnificationAlwaysOnPreferenceControllerTest
Change-Id: I1a25f80131d84ecdd927030e40a18ebb32b7862f