Don't launch placeholder for bottom TaskFragment when waiting for top

1. When the top TaskFragment just appears, we should treat it as active
   even if it is empty, because we are expecting an activity to be
   launched into it soon. Before that happens, we don't want to launch
   placeholder for the activity behind.
2. When a TaskFragment is appeared as empty, set a timeout to remove it
   instead of waiting forever in case the activtiy start failed.

Fix: 230540531
Bug: 219683160
Test: atest WMJetpackUnitTests:SplitControllerTest
Test: atest WMJetpackUnitTests:TaskFragmentContainerTest
Change-Id: Ie05e379bf898b67912a849ad84f9fe7708e289c6