Intent to ship: Media Session API

[Alastor Wu]

*Summary:* Enable Media Session API by default

We've enabled Media Session API on Nightly for a while, and it's time to
enable it on all versions of Firefox. As of Fx82 we intend to turn Media
Session API on by default on all platforms.

However, on Android, this API is still not working until the embedded
applications of GeckoView (eg. Fenix, Firefox Reality) implement GeckoView
MediaSession API
<https://docs.google.com/document/d/1cv7dhLvf7HYr34qNP7SYBNCARBZ2HFbegRsK4PGZHZQ/edit>.

*Bug*: https://bugzilla.mozilla.org/show_bug.cgi?id=1112032

*Bug to turn on by default*:
https://bugzilla.mozilla.org/show_bug.cgi?id=1665496

*Standard:* https://w3c.github.io/mediasession/

*Platform coverage:* All

*Preference:* dom.media.mediasession.enabled

*DevTools bug: *N/A

*Other browsers:* See details here
<https://caniuse.com/mdn-api_mediasession>

*web-platform-tests*: http://w3c-test.org/mediasession/

Cheers,
Alastor