Add back gesture support when split selection active

* Currently this exits the current launcher state
back to homescreen whenever back is performed while
split selection is active.
* Open UX question if that needs to be changed.

Test: Did back gesture from workspace, overview, all apps
http://recall/-/cMb5xTTxhmZtFt04eYnmQj/g0kpGTsqlj0RSt4OfDBkpf
Bug: 295449659
Flag: ENABLE_SPLIT_FROM_WORKSPACE_TO_WORKSPACE
Change-Id: I74fe51aaf4301fb723d2e69e6b1b39d127f492d8