add public method in ContentValues to return set of all keys

why not this method? it is useful and cheap.

Change-Id: I5db96d149b13d26b5359b52515a8eabf033eb000

api/current.xml

@@ -41061,6 +41061,17 @@
 <parameter name="key" type="java.lang.String">
 </parameter>
 </method>
+<method name="keySet"
+ return="java.util.Set&lt;java.lang.String&gt;"
+ abstract="false"
+ native="false"
+ synchronized="false"
+ static="false"
+ final="false"
+ deprecated="not deprecated"
+ visibility="public"
+>
+</method>
 <method name="put"
  return="void"
  abstract="false"

core/java/android/content/ContentValues.java

@@ -446,6 +446,15 @@ public final class ContentValues implements Parcelable {
         return mValues.entrySet();
     }
 
+    /**
+     * Returns a set of all of the keys
+     *
+     * @return a set of all of the keys
+     */
+    public Set<String> keySet() {
+        return mValues.keySet();
+    }
+
     public static final Parcelable.Creator<ContentValues> CREATOR =
             new Parcelable.Creator<ContentValues>() {
         @SuppressWarnings({"deprecation", "unchecked"})