Fixing a bug in maximum users allowed logic on the device.

We need to use CarUserManagerHelper.getMaxSupportedUsers instead of
UserManager.getMaxSupportedUsers() to correctly take user 0 into account
in headles system user model.

Bug: 80197830
Test: atest CarUserManagerHelperTest
Change-Id: I167aa03d2c6ed03dc225cb4686a07395518752b8
2 files changed