checkstyle: enable OneTopLevelClass

The only remaining error was ClipboardCompat, which was unnecessarily
exposing three top-level classes. Make the two implementation classes be
nested, private and static.

app/src/main/java/org/fdroid/fdroid/compat/ClipboardCompat.java

@@ -17,10 +17,8 @@ public abstract class ClipboardCompat {
         return new OldClipboard();
     }
 
-}
-
-@TargetApi(11)
-class HoneycombClipboard extends ClipboardCompat {
+    @TargetApi(11)
+    private static class HoneycombClipboard extends ClipboardCompat {
 
         private final ClipboardManager manager;
 
@@ -39,12 +37,13 @@ class HoneycombClipboard extends ClipboardCompat {
             }
             return text != null ? text.toString() : null;
         }
-}
+    }
 
-class OldClipboard extends ClipboardCompat {
+    private static class OldClipboard extends ClipboardCompat {
 
         @Override
         public String getText() {
             return null;
         }
+    }
 }

config/checkstyle/checkstyle.xml

@@ -62,7 +62,7 @@
         </module>
         <module name="NoLineWrap"/>
 
-        <!--<module name="OneTopLevelClass" />-->
+        <module name="OneTopLevelClass" />
         <module name="OneStatementPerLine" />
 
         <module name="EmptyBlock">