Android BackHandler

This commit continues the work done in #2057.
When hardware back button is pressed, we handle it accordingly:

1. If screen was pushed with `overrideBackPress: true`, we emit `backPress` event
2. Let `BackHandler` handle the event
3. If the event wasn't handled by `BackHandler`, pop the stack
4. let the system handle the event

android/app/src/main/java/com/reactnativenavigation/controllers/NavigationActivity.java

 
     @Override
     public void invokeDefaultOnBackPressed() {
-        super.onBackPressed();
+        if (layout != null && !layout.onBackPressed()) {
+            super.onBackPressed();
+        }
     }
 
     @Override
     public void onBackPressed() {
-        if (layout != null && !layout.onBackPressed()) {
+        if (layout != null && layout.handleBackInJs()) {
+            return;
+        }
+        if (getReactGateway().isInitialized()) {
             getReactGateway().onBackPressed();
+        } else {
+            super.onBackPressed();
         }
     }
 

android/app/src/main/java/com/reactnativenavigation/layouts/BottomTabsLayout.java

 
     @Override
     public boolean onBackPressed() {
-        if (getCurrentScreenStack().handleBackPressInJs()) {
+        if (handleBackInJs()) {
             return true;
         }
 
         }
     }
 
+    @Override
+    public boolean handleBackInJs() {
+        return getCurrentScreenStack().handleBackPressInJs();
+    }
+
     @Override
     public void setTopBarVisible(String screenInstanceId, boolean hidden, boolean animated) {
         for (int i = 0; i < bottomTabs.getItemsCount(); i++) {

android/app/src/main/java/com/reactnativenavigation/layouts/Layout.java

 
     boolean onBackPressed();
 
+    boolean handleBackInJs();
+
     void setTopBarVisible(String screenInstanceId, boolean hidden, boolean animated);
 
     void setTitleBarTitle(String screenInstanceId, String title);

android/app/src/main/java/com/reactnativenavigation/layouts/SingleScreenLayout.java

 
     @Override
     public boolean onBackPressed() {
-        if (stack.handleBackPressInJs()) {
+        if (handleBackInJs()) {
             return true;
         }
 
         }
     }
 
+    @Override
+    public boolean handleBackInJs() {
+        return stack.handleBackPressInJs();
+    }
+
     @Override
     public void destroy() {
         stack.destroy();