Don't bring the Task to front when launching placeholder in background

If the secondary TaskFragment becomes empty, it may trigger to launch
the placeholder activity. If this happens when the Task is in the
background, we don't want the placeholder launch to bring the Task to
the front.

Fix: 232828672
Test: atest WMJetpackUnitTests:SplitControllerTest
Change-Id: Idc159a8c8117e6cc601d7f8650bcfa3c575af076