CTS-test Android Keystore primitives accept empty.

This tests that Android KeyStore Cipher can encrypt empty plaintext,
and that Signature and Mac can generate a signature/MAC for an empty
message. This also tests that empty signatures do not verify.

Bug: 21936191
Change-Id: I145c2f00c97d93387d6d78f9223aa8b144fbdddc
3 files changed