Abstracting out all the Laucher related logic

> Enabling swipe up geture for fallback activity
> Using a preferecens for showing back button

Bug: 75979063
Change-Id: I8bbb44d8e250428c3da9bde57975dc4f952624be

src/com/android/launcher3/BaseDraggingActivity.java

@@ -55,6 +55,8 @@ public abstract class BaseDraggingActivity extends BaseActivity {
     private ActionMode mCurrentActionMode;
     protected boolean mIsSafeModeEnabled;
 
+    private OnStartCallback mOnStartCallback;
+
     @Override
     protected void onCreate(Bundle savedInstanceState) {
         super.onCreate(savedInstanceState);
@@ -85,6 +87,8 @@ public abstract class BaseDraggingActivity extends BaseActivity {
 
     public abstract <T extends View> T getOverviewPanel();
 
+    public abstract View getRootView();
+
     public abstract BadgeInfo getBadgeInfoForItem(ItemInfo info);
 
     public abstract void invalidateParent(ItemInfo info);
@@ -188,4 +192,26 @@ public abstract class BaseDraggingActivity extends BaseActivity {
     protected boolean onErrorStartingShortcut(Intent intent, ItemInfo info) {
         return false;
     }
+
+    @Override
+    protected void onStart() {
+        super.onStart();
+
+        if (mOnStartCallback != null) {
+            mOnStartCallback.onActivityStart(this);
+            mOnStartCallback = null;
+        }
+    }
+
+    public <T extends BaseDraggingActivity> void setOnStartCallback(OnStartCallback<T> callback) {
+        mOnStartCallback = callback;
+    }
+
+    /**
+     * Callback for listening for onStart
+     */
+    public interface OnStartCallback<T extends BaseDraggingActivity> {
+
+        void onActivityStart(T activity);
+    }
 }