Update drop target button alignment across devices

- Partial re-land of http://ag/18520765
- Layout the buttons from center of screen for two panels, or from center of workspace for all other cases

Bug: 229789612
Fix: 231706480
Fix: 232063941
Fix: 232283628
Test: manual
Change-Id: Icbc9e2a19140ce3127d5dfe9f798a8ada336cea8