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

src/com/android/launcher3/model/LoaderTask.java

@@ -690,9 +690,11 @@ public class LoaderTask implements Runnable {
                     break;
 
                 case Favorites.ITEM_TYPE_FOLDER:
+                case Favorites.ITEM_TYPE_APP_PAIR:
                     FolderInfo folderInfo = mBgDataModel.findOrMakeFolder(c.id);
                     c.applyCommonProperties(folderInfo);
 
+                    folderInfo.itemType = c.itemType;
                     // Do not trim the folder label, as is was set by the user.
                     folderInfo.title = c.getString(c.mTitleIndex);
                     folderInfo.spanX = 1;