Document when self-signed certs have invalid signature.

This updates the Javadocs of Android Keystore to explain what key
authorizations are needed for the self-signed cert create at key
generation time to have a valid signature.

Bug: 18088752
Bug: 21777596
Change-Id: Id02425133f094a0c5a02e96f4c63aab7175cba5b
2 files changed