Add AndroidKeyStore provider for KeyStore API

This introduces a public API for the Android keystore that is accessible
via java.security.KeyStore API. This allows programs to store
PrivateKeyEntry and TrustedCertificateEntry items visible only to
themselves.

Future work should include:

* Implement KeyStore.CallbackHandlerProtection parameter to allow the
  caller to request that the keystore daemon unlock itself via the
  system password input dialog.

* Implement SecretKeyEntry once that support is in keystore daemon

Change-Id: I382ffdf742d3f9f7647c5f5a429244a340b6bb0a