Support starting a shortcut into multi-instances split

Support appending MULTIPLE_TASK flag when starting a shotcut into
multi-instances split.
Align the fallback logic of canceling split screen from a shortcut and
an intent to prevent broken split screen.

Bug: 265244780
Test: atest WMShellUnitTests
Test: atest WMShellFlickerTests
Test: verify appended flag from log with the repro steps of the bug
Change-Id: I5942eac71389bcbbef55074fbf127470439fc1e8