Renamed user cmds that change admin privileges.

Also changed then to print an error when failed, and added missing
unit tests.

Note: testRevokeUserAdminFailsForLastFullAdmin() is failing, but
it's not related to this change.

Test: adb shell cmd user grant-admin 10
Test: atest FrameworksMockingServicesTests --test-filter=".*UserManagerServiceMockedTest.*tes.*UserAdmin.*"
Bug: 396197424
Bug: 419105275
Flag: EXEMPT debugging infra

Change-Id: I4d3e8d1be476e0ebe63a888c4cb26bd44d061f42