Bug 1825142 - part2 : use RelaxedAtomicBool for the pref 'media.clear…

…key.test-key-systems.enabled'. r=media-playback-reviewers,aosmond

Differential Revision: https://phabricator.services.mozilla.com/D174988

modules/libpref/init/StaticPrefList.yaml

@@ -9700,7 +9700,7 @@
 
 # Are test specific clearkey key systems enabled and exposed?
 - name: media.clearkey.test-key-systems.enabled
-  type: bool
+  type: RelaxedAtomicBool
   value: false
   mirror: always