Avoid NullPointerException in enableSystemApp if the app is absent.

In enableSystemApp: if the app is absent, throw a IllegalArgumentException
instead of failing with a NullPointerException.

BUG:19321306

Change-Id: I4ec09a0a77d29ca04e8d52f5546c1e4d0f8641e5

services/devicepolicy/java/com/android/server/devicepolicy/DevicePolicyManagerService.java

@@ -5072,6 +5072,10 @@ public class DevicePolicyManagerService extends IDevicePolicyManager.Stub {
             throws RemoteException {
         ApplicationInfo appInfo = pm.getApplicationInfo(packageName, GET_UNINSTALLED_PACKAGES,
                 userId);
+        if (appInfo == null) {
+            throw new IllegalArgumentException("The application " + packageName +
+                    " is not present on this device");
+        }
         return (appInfo.flags & ApplicationInfo.FLAG_SYSTEM) != 0;
     }