Allow devices 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: adb shell settings delete secure accessibility_magnification_magnify_nav_and_ime \
      and see behavior with and without overrides.
Test: WindowStateTests
Flag: com.android.server.accessibility.enable_magnification_magnify_nav_bar_and_ime
Change-Id: I70b74542ff32da283db5d77477793e35b873dbeb