Add UserManagerInternal method to get the main user id. am: 051f571479

Original change: https://android-review.googlesource.com/c/platform/frameworks/base/+/2378889

Change-Id: Iae56d74c01a1b65744b592c73b1009d6e1ae8802
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>