Prevent user stop of protected apps

Paralleling the policy for "force stop," disallow the "task manager"
class of user-invoked application shutdown when the target app is
protected.  Covers both the UI-driven and shell-command-driven paths.

Fixes: 228111137
Test: atest com.android.cts.devicepolicy.DeviceOwnerTest
Change-Id: Idd8b7ef1876f813e97b9f6af85264acb6555335f