Set up Fresco in sample app

Summary:
This looks bad. There are two things I want to follow up on in future
diffs before landing this:

- Make loading the image conditional by adding another button.
- Add a default constructor to the plugin that avoids this crazy bit of boilerplate.

Reviewed By: oprisnik

Differential Revision: D14165569

fbshipit-source-id: 25c5fbaee32e72c6469979cda694cc9f13a92166

android/src/main/java/com/facebook/flipper/plugins/fresco/objecthelper/FlipperObjectHelper.java

@@ -210,5 +210,6 @@ public abstract class FlipperObjectHelper {
     return builder.build();
   }
 
+  @Nullable
   public abstract FlipperArray fromCallerContext(@Nullable Object callerContext);
 }