If a transition happens, discard IME tasks.

When a transtion happens that affects one of the IME target windows, do
not try to restore the bounds of the windows.

Test: atest DesktopImeHandler

Test: trigger IME in a window, then swipe up to home. See no white
surface of the window.

Bug: 388570293

Flag: com.android.window.flags.enable_desktop_ime_bugfix

Change-Id: Idf58d423cefe72593ad222d2a2884faa1f198960