Make multi-instances split opt-in based

Remove appending MULTIPLE_TASK flag when starting an intent and a task
into split screen and leave it to wm-shell to decide whether to append
the flag or not.

Bug: 255224696
Test: atest WMShellFlickerTests
Test: manual check the start intent and task flow
Change-Id: I3d887f8dcea67b12a7e94cef55850d52a79a8d2d