Cleanup Keystore API

Rename confusingly named methods, add userID arguments to all methods
that operate on user state and delete methods that have been replaced by
the onUser* methods.

Some of the old methods have been kept in KeyStore.java in order to ease
the transition of various system packages to the new methods.

Change-Id: Ic271689d62c36d255c5adee26c7abc2e7ed24df5