Add isHeadlessSystemUserMode() to UserManager

- isHeadlessSystemUserMode() returns whether the device is running with
headless system user (go/multi-user-headless-user0)
- RoSystemProperties.MULTIUSER_HEADLESS_SYSTEM_USER is replaced by
isHeadlessSystemUserMode()

Bug: 137755681
Test: Bluetooth service and Network policy manager should work fine with
headless system user. A profile user should be created with headless
system user.

Change-Id: Ic48d98426f7a5ea47dde2008ae51f78855f622e6