Add isMainUser system API.

This adds an API to determine if the context user is the designated
main user on the device. This user may have access to features which
are limited to a single user on the device.

In Android U, this will be the first user to go through setup.

On regular devices, this will be the system user, but on devices in
No Primary User / Headless System User Mode, it will be a different
user.

The isPrimaryUser System API is confusing because it actually always
returns the system user, even in headless mode. That API is deprecated
in this change.

On devices that are upgrading, if they are non-headless, the system user
will be the main user. If they are headless, the full user with the
earliest creation date will be given the flag.

Bug: 256624031
Test: atest UserManagerTest
Change-Id: Ib36f3b372f9bf33cbb097c4af63eb43515ac835b