util: Added @Override annotation where applicable.

Using the @Override annotation is useful for two reasons :

It elicits a warning from the compiler if the annotated method doesn't actually override anything, as in the case of a misspelling.
It improves the readability of the source code by making it obvious that methods are overridden.

src/main/java/com/owncloud/android/utils/FileSortOrderByDate.java

@@ -63,6 +63,7 @@ public class FileSortOrderByDate extends FileSortOrder {
      *
      * @param files list of files to sort
      */
+    @Override
     public List<File> sortLocalFiles(List<File> files) {
         final int multiplier = mAscending ? 1 : -1;
 

src/main/java/com/owncloud/android/utils/FileSortOrderByName.java

@@ -70,6 +70,7 @@ public class FileSortOrderByName extends FileSortOrder {
      *
      * @param files files to sort
      */
+    @Override
     public List<File> sortLocalFiles(List<File> files) {
         final int multiplier = mAscending ? 1 : -1;
 

src/main/java/com/owncloud/android/utils/FileSortOrderBySize.java

@@ -73,6 +73,7 @@ public class FileSortOrderBySize extends FileSortOrder {
      *
      * @param files list of files to sort
      */
+    @Override
     public List<File> sortLocalFiles(List<File> files) {
         final int multiplier = mAscending ? 1 : -1;