[User Switcher] Set default value if setting null.

Copies the logic of user manager which defaults to the device resources
in order to determine if user switcher is enabled by default.

Fixes: 254240785
Test: Added a unit test.
Test: wipe device. Skip setup. Open user switcher, observe that there
should be an add button in the fullscreen user switcher.

Change-Id: I0864de61e7bb9dd7436082f2bf486cf4887a192a