Refactor the cached Authenticator data out of AwtAuthenticator

This makes it easier to swap out authenticator implementations and
yet still rely upon being able to configure at least one Authenticator
instance in the JVM and program it with data obtained from outside
of the user interface.

Change-Id: I8c1a0eb8acee1d306f4c3b40a790b7fa0c3abb70
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>