Improved 'cmd user list-missing-system-packages'.

Changes:

- renamed to report-system-user-package-whitelist-problems
- shows all issues by default
- added option to show just critical issues (--critical-only)
- documented options
- replaced --force by --mode MODE

Test: adb shell cmd user report-system-user-package-whitelist-problems --mode 1 -v --all

Bug: 154112291

Change-Id: I2dfd343d8a20b4241f5e5aa633b8f7222dccb38f