Bug 1772847 - [devtools] Enable devtools.responsive.showUserAgentInput on all channels r=devtools-reviewers,nchevobbe

This preference drives the "Show user agent" feature accessible from the RDM options.
Flipping it to true will make the User agent input visible on all channels by default, but the feature itself was already available everywhere.
You just had to first select "Show user agent" in devedition/beta/release to show the input. I think the feature is useful enough to enable it by default.
Also we run all our tests with the preference forced to true.

Differential Revision: https://phabricator.services.mozilla.com/D150455

browser/app/profile/firefox.js

@@ -2586,13 +2586,8 @@ pref("devtools.responsive.reloadNotification.enabled", true);
 pref("devtools.responsive.touchSimulation.enabled", false);
 // The user agent of the viewport.
 pref("devtools.responsive.userAgent", "");
-
-// Show the custom user agent input only in Nightly.
-#if defined(NIGHTLY_BUILD)
-  pref("devtools.responsive.showUserAgentInput", true);
-#else
-  pref("devtools.responsive.showUserAgentInput", false);
-#endif
+// Show the custom user agent input by default
+pref("devtools.responsive.showUserAgentInput", true);
 
 // Show tab debug targets for This Firefox (on by default for local builds).
 #ifdef MOZILLA_OFFICIAL

devtools/client/responsive/test/browser/head.js

@@ -70,7 +70,6 @@ Services.prefs.setBoolPref(
 );
 // Don't show the setting onboarding tooltip in the test suites.
 Services.prefs.setBoolPref("devtools.responsive.show-setting-tooltip", false);
-Services.prefs.setBoolPref("devtools.responsive.showUserAgentInput", true);
 
 registerCleanupFunction(async () => {
   Services.prefs.clearUserPref(