Signal a rerouted callback if display was set to SingleTaskInstance

singleTaskInstance displays will only contain one task and any attempt
to launch new task will re-route to the default display. Signal a callback
for listeners to handle the case.

Bug: 123642392
Test: atest ActivityManagerMultiDisplayTests#testSingleTaskInstanceDisplay
Change-Id: I16b98d47a798f920a551942d761f07b1df1defc0