Split ShellCommand implementation out of UserManagerService

Moved the shellCommand internal implementation from UserManagerService
to an outer Class UserManagerServiceShellCommand.

Bug: 241245829
Test: adb shell cmd user list
Test: adb shell cmd user report-system-user-package-whitelist-problem
Test: adb shell cmd user set-system-user-mode-emulation --no-restart default
Test: adb shell cmd user is-user-visible 0 cur
BYPASS_INCLUSIVE_LANGUAGE_REASON: existing API
Change-Id: I40e24eb6ee787549540626b3ea698b055d775821