Adding UserManagerHelper class to SettingsLib.

It's a simple wrapper for adding/removing/switching users.

It provides an interface to register listeners for user changes.

Moving it here to be used in CarSettings and SystemUI for the car.

It's not car-specific, so it could be used by anyone who needs it.

Fixes: 74456332
Bug: 74456744
Test: Unit tests in UserManagerHelperTest.java.
Change-Id: I94b3051ed1e4f96d98e5ab2a7a5a7fa75bc5cade