Update package manifest to point to latest ARCore

See changelog: https://docs.unity3d.com/Packages/com.unity.xr.arcore@1.0/changelog/CHANGELOG.html#100-preview19---2018-09-18

Packages/manifest.json

 {
   "dependencies": {
-    "com.unity.xr.arcore": "1.0.0-preview.18",
+    "com.unity.xr.arcore": "1.0.0-preview.19",
     "com.unity.xr.arkit": "1.0.0-preview.14",
     "com.unity.xr.arfoundation": "1.0.0-preview.17"
   }