Ignore source hint rect if it is too small

- When the source hint rect is too small, we can't do a nice transition
  when going home (it would require scaling the surface up to fit the
  rect, which conflicts spatially with the crop down to launcher
  icon/pip), so in this case, just fallback to the overlay

Bug: 190464560
Test: Enter YT Pip, play video, swipe it down to miniplayer then
      enter pip
Change-Id: I44b253ca440f6d2ca0eb20eef3454c5d4e3ae3e9