Separating system and user-installed apps in PS container.

We add a horizontal line separator in PS container which demarcates
user-installed vs system installed apps in private space.
User-installed are shown above the separator, system ones below.

UX Mock: https://www.figma.com/file/K6bIIcG882EiJNjxvSWsFT/V%E2%80%A2-Private-Space?node-id=11546%3A310574&mode=dev
Mock Image: https://photos.app.goo.gl/Wj8sJkS7P7bRbovg8
Demo video: https://photos.app.goo.gl/MBw6HpDnf6PJqUfs8

Bug: 308054233
Flag: ACONFIG com.android.launcher3.Flags.private_space_sys_apps_separation DEVELOPMENT
Test: AlphabeticalAppsListTest
Change-Id: Iad8e289c49a05ac7ef1978bd8e4ebe7aa0add0ca