Bug 1516290 - Disable xdg mime portal in non-flatpack environments by default, r=emilio,stransky

The mime portal breaks checking and setting the default browser.
thus disable it by default, just like D135120 did for printing.

Differential Revision: https://phabricator.services.mozilla.com/D136076

modules/libpref/init/StaticPrefList.yaml

@@ -12635,7 +12635,7 @@
 # Whether to use gtk portal for the mime handler.
 #  - 0: never
 #  - 1: always
-#  - 2: auto (true for flatpak or GTK_USE_PORTAL=1, false otherwise)
+#  - 2: auto (for now only true for flatpak, see bug 1516290)
 - name: widget.use-xdg-desktop-portal.mime-handler
   type: int32_t
   value: 2

widget/gtk/WidgetUtilsGtk.cpp

@@ -113,6 +113,8 @@ bool ShouldUsePortal(PortalKind aPortalKind) {
       case PortalKind::FilePicker:
         return StaticPrefs::widget_use_xdg_desktop_portal_file_picker();
       case PortalKind::MimeHandler:
+        // Mime portal breaks default browser handling, see bug 1516290.
+        autoBehavior = IsRunningUnderFlatpak();
         return StaticPrefs::widget_use_xdg_desktop_portal_mime_handler();
       case PortalKind::Print:
         // Print portal still needs more work, so auto behavior is just when