Add visibility constraints; Fix non-compliant code

Author: emilejq

java/ql/src/Likely Bugs/Concurrency/DateFormatThreadUnsafe.java

@@ -1,5 +1,5 @@
 class DateFormattingThread implements Runnable {
-    private static DateFormat dateF = new SimpleDateFormat("yyyyMMdd");  // Static field declared
+    public static DateFormat dateF = new SimpleDateFormat("yyyyMMdd");  // Static field declared
 
     public void run() {
         for(int i=0; i < 10; i++){

java/ql/src/Likely Bugs/Concurrency/DateFormatThreadUnsafe.ql

@@ -16,6 +16,7 @@ import java
 from Field f, Class dateFormat
 where
   f.isStatic() and
+  (f.isPublic() or f.isProtected()) and
   dateFormat.hasQualifiedName("java.text", "DateFormat") and
   f.getType().(RefType).hasSupertype*(dateFormat) and
   exists(MethodAccess m | m.getQualifier().(VarAccess).getVariable() = f)