cleanup(magnify_ime): Add mag keyboard toggle test coverage for All+Window mode.

As suggested in post-merge comment in
commit 4ea736bd

Bug: 413441693
Test: atest WindowMagnificationSettingsTest
Flag: com.android.server.accessibility.enable_magnification_magnify_nav_bar_and_ime
Change-Id: I9cf341d9aede0ad7719934641191b24102cefe0f