Make DEFAULT_SECURITY_PROVIDER a public constant.
No longer managing key entries, delegating to underlying keystore
instead.
- KeyManager new constructors.
- Public size() and contains() methods added.
Rewrite of init() and save() methods for internally managed
keystore.
Rewrite of save() and load() for internally managed
keystore.
Rewrite of add() to manage internal keystore reference. Add()
implementation also manages storing of public trusted certificates.
Rewrite of remove() for handling internal keystore reference.
Add retrieveKey() implementation for subclasses.
New convenience methods for subclasses:
 - getCertificate()
 - getStorageType()
 - generateKey()
Adds clearPassword() methods for subclass convenience.