Fix unneccesary activity relaunches

When going from fullscreen to non-fullscreen configuration, task config
changes was always non-zero because in fullscreen, task override config
was empty. Instead, use the actual previous configuration to calculate
diff.

Also make recents handle screenLayout changes.

Bug: 26593320
Change-Id: I57633d60b1e0fc4ae506e276410191a44e1fe221