Swap for more framework APIs

Swap CarUserManagerHelper.isUserLimitReached() with
!UserManager.canAddMoreUsers().

Swap CarUserManagerHelper.canForegroundUserAddUsers() with
UserManager.hasUserRestriction().

Implementation is the essentially the same, but UserManager APIs are
better supported across the system.

Bug: 131776619
Test: atest CarSettingsRoboTests
Test: Manually play with UserPicker and changing max user size
Test: adb shell pm set-user-restriction --user 10 no_add_user 1
Then verify add user button does not appear in UserPicker

Change-Id: I8b44ada6423f2a14c11170d890740034c8e9d34f