Move appropriate methods to the JaasRealmIdentity class from JaasSecurityRealm class

JaasRealmIdentity is an inner class of JaasSecurityRealm.

The methods createLoginContext and createCallbackHandler are defined in JaasSecurityRealm but are used only by its inner class JaasRealmIdentity. So they can be moved to the inner class instead for practicality and readability reasons.