a11y: Clean up TestDialogHelper for magnification tests

This moves TestDialogHelper to a separate file, and cleans up a couple
of magnification tests.

Bug: b/397530196
Flag: EXEMPT refactor
Test: SettingsRoboTests:com.android.settings.accessibility.MagnificationModePreferenceControllerTest &&
      SettingsRoboTests:com.android.settings.accessibility.MagnificationCursorFollowingModePreferenceControllerTest
Change-Id: I68e70061b84dacacf95bc05c784677b8d9794754