[automerger skipped] Add UserManagerInternal method to get the main user id. am: 051f571479 am: cc920b547c am: 6f367e02a2 -s ours

am skip reason: Merged-In Ib36f3b372f9bf33cbb097c4af63eb43515ac835b with SHA-1 f8a6722afc is already in history

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

Change-Id: I7fa4ff7584f8d15c625ac26fba1d66d81a224506
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>