Verify IME switch dialog on DualDisplayContent

The current WindowContext mechanism has already covered
the case when ImeContainer reparent to another RootDisplayArea.
When ImeContainer is moved, it sends a onConfigurationChanged
callback, which propagates to the associated WindowContext
and updates its Resources.
Add a test to verify IME switch dialog behavior on display
with dual RootDisplayArea.

fixes: 178462039
Test: atest InputmethodMenuControllerTest

Change-Id: If880ff7207a12d509e8bc37a99583280c793e38c