Fix detaching views rendered under header config component. (#282)

This change fixes the problem when header update would get triggered while the header config subviews are not yet fully unmounted. The root cause of this problem is kind of a mystery, I encouneter a crash when going back using back button from a screen that have custom back button image rendered. Turned out that after implementing removeAll schema the problem no longer occurs. I didn't have enough time to investigate it further but supporting removeAll schema is an improvement regardless so going with this solution for now.

android/src/main/java/com/swmansion/rnscreens/ScreenStackHeaderConfig.java

@@ -243,6 +243,11 @@ public class ScreenStackHeaderConfig extends ViewGroup {
     maybeUpdate();
   }
 
+  public void removeAllConfigSubviews() {
+    mConfigSubviews.clear();
+    maybeUpdate();
+  }
+
   public void addConfigSubview(ScreenStackHeaderSubview child, int index) {
     mConfigSubviews.add(index, child);
     maybeUpdate();