Added Shell command to list system packages that were not configured for exclusion.

Automated tests can check if packages are missing by calling:

  $ adb shell cmd user list-missing-system-packages

(if it returns an empty string, everything is fine)

While developers can use:

  $ adb shell cmd user list-missing-system-packages -v --force

(it lists missing packages even when the system is not on enforce mode)

Test: see above

Bug: 154112291

Change-Id: Iacdf121786f238059f61e83a257c07463c4097c5