Show IME-related system IME dialogs over windows owned by the IME

The IME-related system dialogs should be placed over any window
owned by the current IME. However, the framework has taken only
the main window of the current IME has been taken into account.
As a result, the system has not been able to show its IME-related
dialog properly layer when the IME is showing its own dialog.

This CL addresses the above issue.

BUG: 5809660
Change-Id: I205321c7ed42516f36a4bf29dd6a159ac34fd5ec