Add SecretKeyFactory backed by AndroidKeyStore.

This factory provides a way to obtain information about a SecretKey
backed by AndroidKeyStore. The information is provided in a form of an
instance of KeyStoreKeySpec class.

EXAMPLE

SecretKeyFactory factory = SecretKeyFactory.getInstance(
        key.getAlgorithm(), "AndroidKeyStore");
KeyStoreKeySpec keySpec =
    factory.getKeySpec(key, KeyStoreKeySpec.class);

Bug: 18088752
Change-Id: I26c9dd544f80230fe7039501eeb471eaf875452b