Hide PluginVmLicenseKey device cloud policy value

Bug: 938252
Change-Id: I34600b86d73a1bc8c1db04af717216e406e02af9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1503453Reviewed-by: Julian Pastarmov <pastarmovj@chromium.org>
Commit-Queue: Olya Kalitova <okalitova@chromium.org>
Cr-Commit-Position: refs/heads/master@{#638077}

components/policy/resources/policy_templates.json

@@ -14651,7 +14651,10 @@
     {
       'name': 'PluginVmLicenseKey',
       'type': 'string',
-      'schema': { 'type': 'string' },
+      'schema': {
+        'sensitiveValue': True,
+        'type': 'string'
+      },
       'supported_on': ['chrome_os:73-'],
       'device_only': True,
       'features': {