Bug 1648100 - enable media control by default. r=chunmin

We're going to enable media control on Fx81.

Differential Revision: https://phabricator.services.mozilla.com/D86991

modules/libpref/init/StaticPrefList.yaml

@@ -7522,7 +7522,7 @@
 
 - name: media.hardwaremediakeys.enabled
   type: bool
-  value: @IS_NIGHTLY_BUILD@
+  value: true
   mirror: always
 
 # If this pref is on, then `media.mediacontrol.stopcontrol.timer.ms` would take