Fix WindowContext cannot receive global config

... change on secondary display.

Previously we listen to #onRequestedOverrideConfigurationChanged() to
receive config changes, which only propagate callbacks if there's a
requested override configuration. This CL changes to listen to
 #onMergedOverrideConfigurationChanged() to ensure every config change
will propagate to WindowContext.

fixes: 185190979
Test: atest WindowContextTests

Change-Id: I0299a2795bd0b009ec2291a36aa1c87e163222b7