CertExams.com released CCNA exam for android, Check out the link below:
https://play.google.com/store/apps/details?id=com.anandsoft.practicetestsccna
Now it is possible to buy one test at a time, rather than buying all the tests in one go. The free version will have 60 questions with answers and flash card explanation. Each test will have 125 questions, and one can buy one test at a time using inapp purchase option.