Allow devices to override default behavior of magnify NavBar and IME

This change allows device configuration to change the default behavior
of fullscreen magnification magnifying navigation bar and IME windows.

Bug: 357764515
Test: MagnifyNavAndImePreferenceControllerTest
Flag: com.android.server.accessibility.enable_magnification_magnify_nav_bar_and_ime
Change-Id: I80e0a9a90c095a6ee230f84137c7b6216a75c45a