Use the secondary display for InputMethodMenuControllerTest

After [1] merged, mDisplayContent is assigned to mDefaultDisplay
if the test is not marked as @UseTestDisplay.
However, the CL to introduce testGetSettingsContext wasn't aware of
the change and didn't test the behavior of switching displays.

This CL creates a test display and also mock some methods to
verify the behavior.

[1]: f8d8aea4

fixes: 173491267
Test: atest InputMethodMenuControllerTest

Change-Id: Ie5ee0f1c781a20d0b97b1a948cb44dcb120b81ae