autotestPrivate has a number of functions that are related to authentication, a number of functions related to input devices (touchpad, mouse, touchscreen), and some random functions.

We would like to pull the authentication related functions into authPrivate, for use by whitelisted login screen apps.

We should also separate out the input device related functions into inputDeviceControlPrivate.

These are the auth-related functions:
https://cs.chromium.org/chromium/src/chrome/common/extensions/api/autotest_private.idl?l=67-81

Since autotestPrivate methods are used in the chromeos repository, this has to be done in 3 stages.
1. Introduce a copy of these functions in authPrivate.
2. Switch code in the chromeos repo to use authPrivate.
3. Deprecate these functions from autotestPrivate.