Use hierarchy focusability to pause the minimized tile

When split-screen minimizes, it should pause whatever is
in the minimized tile.

Bug: 133381284
Test: updated focusability test and existing tests pass.
      Also manually open split minimized and check that
      the primary stack is PAUSED.
Change-Id: I91cd6634f7078a3007d80bb924de508869b6a2d6