Showcase: Postprocessor example: use fitCenter

Summary: In order to test postprocessors, it makes sense to see the whole image. For example, for the circular postprocessor, the full circle should be visible independent of the image aspect ratio.

Reviewed By: dmitry-voronkevich

Differential Revision: D6247935

fbshipit-source-id: 6702f0f27033a8497d47ac5a182ae3119fb0aa8f

Author: oprisnik

samples/showcase/src/main/res/layout/fragment_imagepipeline_postprocessor.xml

@@ -1,6 +1,7 @@
 <?xml version="1.0" encoding="utf-8"?>
 <LinearLayout
     xmlns:android="http://schemas.android.com/apk/res/android"
+    xmlns:fresco="http://schemas.android.com/apk/res-auto"
     android:layout_width="match_parent"
     android:layout_height="match_parent"
     android:layout_marginLeft="@dimen/activity_horizontal_margin"
@@ -25,6 +26,7 @@
       android:layout_marginTop="@dimen/margin_medium"
       android:background="@color/primary"
       android:padding="@dimen/padding_small"
+      fresco:actualImageScaleType="fitCenter"
       />
 
   <Button