Fix/java 1.5 support

Android 2.2 devices are based on Java 1.5.

Two following APIs not available on Android 2.2:
- The String.isEmpty() method was introduced on Java 1.6. The 2.2 version of Android is based on Java 5. It was added only with API level 9 (Gingerbread). Replaced the isEmpty() call with “length() == 0”.
- The copyOfRange() method is another java 1.6 API that is being used by MailCheck. Replaced it with System.arraycopy()