Add UserManagerInternal method to get the main user id. am: 051f571479 am: cc920b547c

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

Change-Id: I9fb10b4098e33563b94222bba5aaa91520fd4acb
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>