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/logging/StatsLogManager.java

@@ -641,11 +641,17 @@ public class StatsLogManager implements ResourceBasedOverride {
         @UiEvent(doc = "User has swiped upwards from the gesture handle to show transient taskbar.")
         LAUNCHER_TRANSIENT_TASKBAR_SHOW(1331),
 
+        @UiEvent(doc = "User has clicked an app pair and launched directly into split screen.")
+        LAUNCHER_APP_PAIR_LAUNCH(1374),
+
+        @UiEvent(doc = "User saved an app pair.")
+        LAUNCHER_APP_PAIR_SAVE(1456),
+
         @UiEvent(doc = "App launched through pending intent")
-        LAUNCHER_APP_LAUNCH_PENDING_INTENT(1394),
-        ;
+        LAUNCHER_APP_LAUNCH_PENDING_INTENT(1394)
 
         // ADD MORE
+        ;
 
         private final int mId;