Support back gesture to split pair in background

Split screen will be dismissed right after both sides of the split
were occluded. So if users try to use a back-gesture to go back to
the previous split pair, it’ll be back to home instead of going
back to the split pair. This CL support the split pair in background
to make split experience better.

Bug: 236317871
Test: atest com.android.wm.shell.SplitLayoutTests
Change-Id: I1ead65011ced82e1faa24ccfbc8bf474828fc7dc