changed String.isEmpty to TextUtils.isEmpty

The build.xml indicates Java 1.5 support but isEmpty is only for Java 1.6+. 
See http://docs.oracle.com/javase/6/docs/api/java/lang/String.html#isEmpty()

When I ran in API level 8, there is an error for missing java.lang.String.isEmpty.

Now it should be able to work in Java 1.5