Changed output of get-main-user.

It will be just the user id when the device supports it, and "None"
when it doesn't.

Test: adb shell cmd user get-main-user
Fixes: 271295369

Change-Id: I1d4ad4765cddade369ce1d7ce515fe5907d84932

services/core/java/com/android/server/pm/UserManagerServiceShellCommand.java

@@ -530,10 +530,10 @@ public class UserManagerServiceShellCommand extends ShellCommand {
         PrintWriter pw = getOutPrintWriter();
         final int mainUserId = mService.getMainUserId();
         if (mainUserId == UserHandle.USER_NULL) {
-            pw.println("Couldn't get main user.");
+            pw.println("None");
             return 1;
         }
-        pw.println("Main user id: " + mainUserId);
+        pw.println(mainUserId);
         return 0;
     }