Improvements on UserManagerService ShellCommand implementation (1/4).

These changes will make it easier to extend / mantain it:

- Un-hardcoded parameters and arguments.
- Adds empty line after each command on help.

BYPASS_INCLUSIVE_LANGUAGE_REASON: existing API

Test: adb shell cmd user
Bug: 203885212

Change-Id: If095ff3fc7f57d3c080644cb1de62c32187ae31e