Bump checkstyle to 6.16

Due to fixes and improvements, some new checkstyle changes are due.

F-Droid/build.gradle

@@ -192,7 +192,7 @@ android {
 }
 
 checkstyle {
-    toolVersion = '6.15'
+    toolVersion = '6.16'
 }
 
 task checkstyle(type: Checkstyle) {

F-Droid/src/org/fdroid/fdroid/data/DBHelper.java

@@ -110,7 +110,7 @@ class DBHelper extends SQLiteOpenHelper {
 
     private final Context context;
 
-    public DBHelper(Context context) {
+    DBHelper(Context context) {
         super(context, DATABASE_NAME, null, DB_VERSION);
         this.context = context;
     }

F-Droid/src/org/fdroid/fdroid/data/InstalledAppCacheUpdater.java

@@ -23,7 +23,7 @@ import java.util.Map;
  * TODO: The content providers are not thread safe, so it is possible we will be writing
  * to the database at the same time we respond to a broadcasted intent.
  */
-public class InstalledAppCacheUpdater {
+public final class InstalledAppCacheUpdater {
 
     private static final String TAG = "InstalledAppCache";
 

F-Droid/src/org/fdroid/fdroid/views/swap/SwapWorkflowActivity.java

@@ -774,8 +774,7 @@ public class SwapWorkflowActivity extends AppCompatActivity {
                     public void run() {
                         new SwapDebug().logStatus();
                     }
-                },
-                1000
+                }, 1000
             );
         }
     }

Privileged-Extension/build.gradle

@@ -36,7 +36,7 @@ android {
 }
 
 checkstyle {
-    toolVersion = '6.15'
+    toolVersion = '6.16'
 }
 
 task checkstyle(type: Checkstyle) {