Align AndroidKeyStore API with user auth API.

This simplifies the AndroidKeyStore API around user authentication: no
more explicit control over which user authenticators are bound to
which keys.

User-authenticated keys with timeout are unlocked by whatever unlocks
the secure lock screen (currently, password/PIN/pattern or
fingerprint). User-authenticated keys that need authentication for
every use are unlocked by fingerprint only.

Bug: 20526234
Bug: 20642549
Change-Id: I1e5e6c988f32657d820797ad5696797477a9ebe9