UserInfo: FULL and SYSTEM flags

Introduces two new UserInfo flags:

SYSTEM - refers to the system user, i.e. user 0. Although
UserManagerService already knows which user is the system, this flag
will be useful in other contexts.

FULL - refers to a non-profile human user.

Right now, for every user, one of the following flag combos are true:
SYSTEM (user 0 on a headless-user-0 device)
SYSTEM | FULL (user 0 on a regular device)
FULL (secondary user)
MANAGED_PROFILE (profile users)

Test: Manual verification via "pm list users" of new flags when adding a new user
Test: Manual verification via "pm list users" of new flags for pre-existing users
Test: Later cls will test the functionality as it is introduced
Bug: 134605778
Change-Id: Ife92bfa8a91cc4567d825da8e6a1b3894b088b22