App Pairs: Implement save, inflate, launch, and delete

This is the third of several patches implementing the App Pairs feature behind a flag.

This patch includes:
- AppPairIcon and associated XML. Actual icon asset is placeholder for now
- Ability to launch split pair on click
- Icon can be moved around, incl. to Taskbar
- App pair can be deleted by dragging to "Remove" drop zone
- Icon persists on Launcher reload

Change-Id: I88aec6fbc814be98f9ef048bbc5af889d0797970
Flag: ENABLE_APP_PAIRS (set to false)
Bug: 274835596
Test: Not included in this CL, but will follow