flow: Fix the messageInput and topicInput types.

Until now Flow have not checked `ComposeBox.android.js` so it
did not catch a mistake of ours. The two values are not of type
`TextInput`, which is a type that refers to the rendered component,
but are references to the native component that is named as
`InternalTextInputType` inside the React Native source code.

This type is not exported though so we can not access it.
Also, it does not seem to be complete, missing the `focus` function.

src/compose/ComposeBox.android.js

@@ -95,8 +95,8 @@ class ComposeBox extends PureComponent<Props, State> {
   props: Props;
   state: State;
 
-  messageInput: ?TextInput = null;
-  topicInput: ?TextInput = null;
+  messageInput: any = null;
+  topicInput: any = null;
 
   static contextTypes = {
     styles: () => null,