Suppress the warning toast if the preferredDisplay was set to singleTask

singleTaskInstance displays will only contain one task and any attempt
to launch new task will re-route to the default display.

Bug: 123642392
Test: atest ActivityManagerMultiDisplayTests#testSingleTaskInstanceDisplay
Change-Id: If782a121c5a303b6bfa9f23da59c1a62ef242c7f