Make multi-instances split opt-in based

Split screen was by default launching multi-instnaces if users launch
the same component into both sides of the split. This limited the
behavior to packages declaired in
config_componentsSupportMultiInstancesSplit.

Bug: 255224696
Test: atest SplitScreenControllerTests
Test: atest WMShellFlickerTests
Change-Id: I6b03007859b37f60d5e7789142462ff26e1c7b8f