Enable short shelf for split shade

Bug: 213480466
Test: NotificationShelfTest
Test: swipe down on lockscreen notifications, let go
   => short shelf expands, retracts width
Test: swipe down on lockscreen notifications to go to unlocked shade
   => short shelf expands to full width
Test: expand notification on lockscreen
   => short shelf expands to full width as shade opens
Test: tap hidden/visible part of shelf
   => only visible part of shelf launches full shade
Test: the above in LTR/RTL
Change-Id: I3f7d488531be3af544a41f4fab496b3ccc137f24