Fixes commons-app#3575 (commons-app#3602)

* Donot show random colors as placeholders for contributions items
* Theme contributionsItem background as mainBackground from attributes

Author: ashishkumar468

app/src/main/java/fr/free/nrw/commons/contributions/ContributionViewHolder.java

@@ -50,8 +50,6 @@ public class ContributionViewHolder extends RecyclerView.ViewHolder {
     public void init(int position, Contribution contribution) {
         this.contribution = contribution;
         this.position = position;
-        imageView.getHierarchy().setPlaceholderImage(new ColorDrawable(
-            Color.argb(100, random.nextInt(256), random.nextInt(256), random.nextInt(256))));
         String imageSource = chooseImageSource(contribution.thumbUrl, contribution.getLocalUri());
         if (!TextUtils.isEmpty(imageSource)) {
             final ImageRequest imageRequest =

app/src/main/res/layout/layout_contribution.xml

@@ -23,6 +23,7 @@
         android:layout_width="match_parent"
         android:layout_height="@dimen/very_large_height"
         app:actualImageScaleType="centerCrop"
+        android:background="?attr/mainBackground"
         fresco:placeholderImage="@drawable/ic_image_black_24dp"
         />