Implement push into modal

closes #2487

AndroidE2E/app/src/androidTest/java/com/reactnativenavigation/e2e/androide2e/ModalsTest.java

 		elementByText("DISMISS ALL MODALS").click();
 		assertMainShown();
 	}
+
+	@Test
+    public void pushIntoModal() throws Exception {
+        elementByText("SHOW MODAL").click();
+        elementByText("PUSH SCREEN").click();
+        assertExists(By.text("Pushed from modal"));
+    }
 }

lib/android/app/src/main/java/com/reactnativenavigation/viewcontrollers/ModalStack.java

 		return null;
 	}
 
-	private static class Modal {
-		private final ViewController viewController;
+	@Nullable
+    ViewController findControllerById(String id) {
+        Modal modal = findModalByContainerId(id);
+        return modal != null ? modal.viewController : null;
+    }
+
+    private static class Modal {
+		public final ViewController viewController;
 		private final Dialog dialog;
 
 		Modal(final ViewController viewController) {

lib/android/app/src/main/java/com/reactnativenavigation/viewcontrollers/Navigator.java

 
 import android.app.Activity;
 import android.support.annotation.NonNull;
+import android.support.annotation.Nullable;
 import android.view.ViewGroup;
 import android.widget.FrameLayout;
 
 			promise.reject(new Throwable("Nothing to pop"));
 		}
 	}
+
+    @Nullable
+    @Override
+    public ViewController findControllerById(String id) {
+        ViewController controllerById = super.findControllerById(id);
+        return controllerById != null ? controllerById : modalStack.findControllerById(id);
+    }
 }

lib/android/app/src/main/java/com/reactnativenavigation/viewcontrollers/ViewController.java

 		this.parentController = parentController;
 	}
 
-    void performOnParentStack(Task<StackController> task) {
+    boolean performOnParentStack(Task<StackController> task) {
 	    if (parentController instanceof StackController) {
             task.run((StackController) parentController);
+            return true;
         }
+        if (this instanceof StackController) {
+	        task.run((StackController) this);
+            return true;
+        }
+        return false;
     }
 
     void performOnParentStack(Task<StackController> accept, Runnable  reject) {
-        if (parentController instanceof StackController) {
-            accept.run((StackController) parentController);
-        } else {
+        if (!performOnParentStack(accept)) {
             reject.run();
         }
     }

lib/android/app/src/test/java/com/reactnativenavigation/viewcontrollers/NavigatorTest.java

 			}
 		});
 	}
+
+	@Test
+    public void pushIntoModal() throws Exception {
+        StackController stackController = newStack();
+        stackController.push(child1);
+        uut.showModal(stackController, new MockPromise());
+        uut.push(stackController.getId(), child2);
+        assertIsChildById(stackController.getView(), child2.getView());
+    }
 }

playground/src/containers/ModalScreen.js

     this.onClickDismissAllPreviousModals = this.onClickDismissAllPreviousModals.bind(this);
     this.onClickDismissFirstInStack = this.onClickDismissFirstInStack.bind(this);
     this.onClickDismissAllModals = this.onClickDismissAllModals.bind(this);
+    this.onClickPushScreen = this.onClickPushScreen.bind(this);
   }
 
   render() {
         <Button title="Dismiss Modal" testID={testIDs.DISMISS_MODAL_BUTTON} onPress={this.onClickDismissModal} />
         <Button title="Dismiss Unknown Modal" testID={testIDs.DISMISS_UNKNOWN_MODAL_BUTTON} onPress={this.onClickDismissUnknownModal} />
         <Button title="Dismiss All Modals" testID={testIDs.DISMISS_ALL_MODALS_BUTTON} onPress={this.onClickDismissAllModals} />
+        <Button title="Push screen" testID={testIDs.PUSH_BUTTON} onPress={this.onClickPushScreen} />
         {this.getPreviousModalId() ? (<Button title="Dismiss Previous Modal" testID={testIDs.DISMISS_PREVIOUS_MODAL_BUTTON} onPress={this.onClickDismissPreviousModal} />) : undefined}
         {this.props.previousModalIds ? (<Button title="Dismiss ALL Previous Modals" testID={testIDs.DISMISS_ALL_PREVIOUS_MODAL_BUTTON} onPress={this.onClickDismissAllPreviousModals} />) : undefined}
         {this.props.previousModalIds ? (<Button title="Dismiss First In Stack" testID={testIDs.DISMISS_FIRST_MODAL_BUTTON} onPress={this.onClickDismissFirstInStack} />) : undefined}
     Navigation.dismissAllModals();
   }
 
+  onClickPushScreen() {
+    Navigation.push(this.props.containerId, {
+      name: `navigation.playground.TextScreen`,
+      passProps: {
+        text: 'Pushed from modal'
+      }
+    });
+  }
+
   getModalPosition() {
     return (this.props.modalPosition || 1);
   }