Don't try to remove user 0 in CTS tests

There was a change in the usermanager that results in failure
when a non-removable/stoppable user is attempted to be removed.
Adjusting CTS test accordingly.

Bug: 27550623
Change-Id: I88409a632e333e863bb2df37e4bc7751246cd07c
diff --git a/hostsidetests/devicepolicy/src/com/android/cts/devicepolicy/BaseDevicePolicyTest.java b/hostsidetests/devicepolicy/src/com/android/cts/devicepolicy/BaseDevicePolicyTest.java
index 71f11a7..80a94c7 100644
--- a/hostsidetests/devicepolicy/src/com/android/cts/devicepolicy/BaseDevicePolicyTest.java
+++ b/hostsidetests/devicepolicy/src/com/android/cts/devicepolicy/BaseDevicePolicyTest.java
@@ -166,7 +166,9 @@
     }
 
     protected void removeUser(int userId) throws Exception  {
-        assertTrue("Couldn't remove user", getDevice().removeUser(userId));
+        if (listUsers().contains(userId) && userId != 0) {
+            assertTrue("Couldn't remove user", getDevice().removeUser(userId));
+        }
     }
 
     protected void removeTestUsers() throws Exception {