Fix package shell command

Fixes: 131230523
Fixes: 141135574
Fixes: 137410612
Test: Manual; 'adb shell cmd package help' shows output for "install-existing"
Test: Manual; 'adb shell cmd package help' shows output for "list users"
Test: Manual; 'adb shell cmd package list packages --user current' shows output for the current user
Change-Id: Id1ead44b4b8f1d01f805c0d317d77e93069095b6