title: AndroidKeystore on 4.1 API
body: <p>I am using <code>AndroidKeystore</code> as the provider in my current project to store private keys securely.
  Now I want to make my app available for older Android releases down to 4.1 (API version 16).
  Therefore I need to find an alternative for the <code>AndroidKeystore</code> but can hardly find any usable solution.
  Is there any recommendation how to implement/emulate/replace the <code>AndroidKeystore</code> functionality on these older versions?</p>
  
  <p>Additionally the <code>KeyPairGeneratorSpec</code> is available since v18. What would be the correct replacement on older versions?</p>