Allow caller dismissing split to specify top stack

When starting a non-resizeable activity while in split-mode it is
important the non-resizeable activity causing the dismisal of the split
end up on top. This change allows the callers of dismiss split to
specify which stack will be topmost.

Fixes: 152943129
Test: atest testNonResizeableWhenAlreadyInSplitScreenPrimary testNonResizeableWhenAlreadyInSplitScreenSecondary
Change-Id: Iefba54183273f1d075fbeacdcd87133eb1f2e3ad