Disallow userLeaving when split drag drop

We should avoid setting mUserLeaving=true when
bringing task to front that disallow entering PiP
due to the launching task coming to front.

Bug: 336477473
Test: repro the steps in the bug report
Change-Id: I94377538ab922b2ca03f295a847a8b8e4d1904df
(cherry picked from commit 86860b8e659b51dd48fffb3197a30d15b8482d3f)
1 file changed