java: understand more initializers

Whne a fiels is assigned a safe type in a constructor,
that field is not exposed.

java/ql/lib/semmle/code/java/ConflictingAccess.qll

@@ -63,12 +63,23 @@ class ExposedField extends Field {
     not this.getType() instanceof LockType and
     // field is not thread-safe
     not isThreadSafeType(this.getType()) and
-    not isThreadSafeType(this.getInitializer().getType()) and
+    not isThreadSafeType(initialValue(this).getType()) and
     // the initializer guarantees thread safety
-    not isThreadSafeInitializer(this.getInitializer())
+    not isThreadSafeInitializer(initialValue(this))
   }
 }
 
+/**
+ * Gets the initial value for the field `f`.
+ * This is either a static initializer or an assignment in a constructor.
+ */
+Expr initialValue(Field f) {
+  result = f.getInitializer()
+  or
+  result = f.getAnAssignedValue() and
+  result.getEnclosingCallable() = f.getDeclaringType().getAConstructor()
+}
+
 /**
  * A field access that is exposed to potential data races.
  * We require the field to be in a class that is annotated as `@ThreadSafe`.