Implement platform support for persisting keys and certificates (Java/Anrdoid)

I understand that Java and Android could share the same code so I create a single ticket.

Reference:

Key API: https://github.com/couchbase/couchbase-lite-core/blob/master/Crypto/PublicKey.hh#L113-L169

Certificate API: https://github.com/couchbase/couchbase-lite-core/blob/master/Crypto/Certificate.hh#L186-L189 (See https://github.com/couchbase/couchbase-lite-core/pull/959 for potential update)

Apple Implementation: https://github.com/couchbase/couchbase-lite-core/blob/master/Crypto/PublicKey%2BApple.mm

Some related pending PRs for Apple:

https://github.com/couchbase/couchbase-lite-core/pull/956

https://github.com/couchbase/couchbase-lite-core/pull/958

https://github.com/couchbase/couchbase-lite-core/pull/959

As Java/Android will require to call back to the Java's KeyStore to generate keys and save/load/delete the keys and certificate, we might need an abstract object representing the KeyStore to be passed from JNI code to the native code.