fix some warnings for lint

Signed-off-by: Abdourahamane Boinaidi <abdourahamane.boinaidi@infomaniak.com>

src/main/java/com/owncloud/android/ui/adapter/OCFileListAdapter.java

@@ -31,7 +31,6 @@ import android.content.ContentValues;
 import android.content.Context;
 import android.content.res.Resources;
 import android.graphics.Bitmap;
-import android.graphics.Color;
 import android.graphics.LinearGradient;
 import android.graphics.PorterDuff;
 import android.graphics.Shader;
@@ -614,7 +613,7 @@ public class OCFileListAdapter extends RecyclerView.Adapter<RecyclerView.ViewHol
         setThumbnail(file, thumbnailView, user, storageManager, asyncTasks, gridView, context, null, null);
     }
 
-    public static void setThumbnail(OCFile file,
+    private static void setThumbnail(OCFile file,
                                     ImageView thumbnailView,
                                     User user,
                                     FileDataStorageManager storageManager,

src/main/res/layout/grid_image.xml

@@ -42,8 +42,9 @@
                 android:id="@+id/thumbnail_shimmer"
                 android:layout_width="match_parent"
                 android:layout_height="match_parent"
-                android:background="@color/grey_200"
                 android:layout_margin="@dimen/standard_eigth_padding"
+                android:background="@color/grey_200"
+                android:contentDescription="@null"
                 android:scaleType="centerCrop"
                 android:visibility="gone" />