Create PrintDiscoverySession for future print services

If we currently don't have any print services we still want to register
a print observer so that when in the future a new service comes along we
can connect this service to the observer.

Bug: 25666802
Change-Id: I0cc28e92e0000fc9b5372733390095f3a8b19bc9

services/print/java/com/android/server/print/UserState.java

@@ -343,9 +343,7 @@ final class UserState implements PrintSpoolerCallbacks, PrintServiceCallbacks {
     public void createPrinterDiscoverySession(IPrinterDiscoveryObserver observer) {
         synchronized (mLock) {
             throwIfDestroyedLocked();
-            if (mActiveServices.isEmpty()) {
-                return;
-            }
+
             if (mPrinterDiscoverySession == null) {
                 // If we do not have a session, tell all service to create one.
                 mPrinterDiscoverySession = new PrinterDiscoverySessionMediator(mContext) {