Add UserManagerInternal method to get the main user id.

This adds a method to get the id of the main user on the device.

Currently this always returns the id of the system user.

Bug: 256624031
Test: atest VpnManagerServiceTest (with topic)
Merged-In: Ib36f3b372f9bf33cbb097c4af63eb43515ac835b
Change-Id: I4aa0feb68083460c6aef917337e8bfc09a6788d4
2 files changed