Check that sysui states before allowing back gesture

- Ensure not in immersive mode, and also that the notification panel is
  not expanded

Bug: 130340004
Bug: 130417141
Test: Manual, ensure back doesn't work when notification panel is down or
      app is in immersive mode

Change-Id: I29f5cd80cc89b28802ca8b2c9e2956ae5b3b1151