Allow QuickstepTransitionManager to be overridden

This allows for custom animations in classes inheriting from
QuickstepLauncher

Bug: 260637266
Test: flashed device and verified animation
Change-Id: I21fa112851e9ce43037151fc4c2fd21e90056712

quickstep/src/com/android/launcher3/uioverrides/QuickstepLauncher.java

@@ -238,7 +238,7 @@ public class QuickstepLauncher extends Launcher {
         mActionsView.updateDimension(getDeviceProfile(), overviewPanel.getLastComputedTaskSize());
         mActionsView.updateVerticalMargin(DisplayController.getNavigationMode(this));
 
-        mAppTransitionManager = new QuickstepTransitionManager(this);
+        mAppTransitionManager = buildAppTransitionManager();
         mAppTransitionManager.registerRemoteAnimations();
         mAppTransitionManager.registerRemoteTransitions();
 
@@ -307,6 +307,13 @@ public class QuickstepLauncher extends Launcher {
         return mHotseatPredictionController;
     }
 
+    /**
+     * Builds the {@link QuickstepTransitionManager} instance to use for managing transitions.
+     */
+    protected QuickstepTransitionManager buildAppTransitionManager() {
+        return new QuickstepTransitionManager(this);
+    }
+
     @Override
     protected QuickstepOnboardingPrefs createOnboardingPrefs(SharedPreferences sharedPrefs) {
         return new QuickstepOnboardingPrefs(this, sharedPrefs);