Moving manifest initialization to RoboApplication instead of a separate rule

This makes the behavior same as instrumentation test

Bug: 370080120
Test: Presubmit
Flag: EXEMPT test fix
Change-Id: Id4183c91f3951748b657b583fde8860abe55d3a8

tests/multivalentTests/src/com/android/launcher3/model/DefaultLayoutProviderTest.java

@@ -39,7 +39,6 @@ import com.android.launcher3.util.LauncherModelHelper;
 
 import org.junit.After;
 import org.junit.Before;
-import org.junit.Rule;
 import org.junit.Test;
 import org.junit.runner.RunWith;
 
@@ -50,8 +49,6 @@ import org.junit.runner.RunWith;
 @RunWith(AndroidJUnit4.class)
 public class DefaultLayoutProviderTest {
 
-    @Rule public ModelTestRule rule = new ModelTestRule();
-
     private LauncherModelHelper mModelHelper;
     private LauncherModelHelper.SandboxModelContext mTargetContext;