Don't start recents multiple times when entering split-screen

ActivityStack.setWindowingMode() takes care of starting the recents
activity when split-screen is activated so we don't need to do this
all over the place anymore.

Change-Id: I62153c4125744e9da533b6293e91c0658868bd1b
Fixes: 68952186
Test: ActivityManagerSplitScreenTests#testLaunchToSideAndBringToFront