Fix SplitScreenTests#testNoUserLeaveHintOnMultiWindowModeChanged flaky

Launch another activity on secondary stack to make sure test activity
could be in normal split mode and focused.

Bug: 157957860
Test: atest SplitScreenTests#testNoUserLeaveHintOnMultiWindowModeChanged --rerun-until-failure
Change-Id: Id403c5b94b3bc46d1717e6f17b8ec9800d2346bf