cros: Fix login crash under mash due to multiple ash instances

The ash service was being started twice because it was not marked as
a "singleton" service. Service connections with different user ids
could result in the service starting twice in the same process.

This caused CHECK failures in bluez::BluezDBusManager::Initialize
and desktopui_MashLogin to fail in the autotest lab.

Fix by marking the ash and ui services as singlton services, which
they should have been in the first place.

Bug: 836311
Test: existing autotest desktopui_MashLogin
Change-Id: I066692b19bb5d89c75e46c79675024740f42f844
Reviewed-on: https://chromium-review.googlesource.com/1026202Reviewed-by: Ken Rockot <rockot@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553235}

ash/manifest.json

@@ -61,6 +61,7 @@
         "catalog": [ "directory" ],
         "local_state": [ "pref_client" ],
         "quick_launch_app": [ "mash:launchable" ],
+        "service_manager": [ "service_manager:singleton" ],
         "ui": [ "display_dev", "window_manager", "video_detector" ],
         "touch_hud_app": [ "mash:launchable" ],
         "data_decoder": [ "image_decoder" ]

services/ui/manifest.json

@@ -81,7 +81,7 @@
       "requires": {
         "*": [ "app" ],
         "catalog": [ "directory" ],
-        "service_manager": [ "service_manager:all_users" ],
+        "service_manager": [ "service_manager:singleton" ],
         "ui": [ "ozone" ],
         "viz": [ "viz_host" ]
       }