Fix FlutterOutline service infinite subscriptions (#8057)

Fixes an issue where we never unsubscribe outline listeners.

The rub is that `getLocalFileUri` is being called on a path that is
already a URI and is producing a bogus path which has no associated
listener registered (because it’s bogus).

In general the issue here is a symptom of URIs being encoded in strings
that are never validated.

The quick fix is to not “uri-ify” an already valid UR string. Longer
term we should consider a deeper audit of places we’re passing URIs
around. It’s possible there are other related issues. If/when we do
this, Alex shared some [interesting thoughts from his work on the Dart
plugin](#7980 (comment))
that’d be worth synthesizing.

Fixes: #7980

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Author: pq

flutter-idea/src/io/flutter/editor/ActiveEditorsOutlineService.java

@@ -132,8 +132,7 @@ private void updateActiveEditors() {
       }
 
       for (final String path : obsoletePaths) {
-        final String filePathOrUri = getAnalysisServer().getAnalysisService().getLocalFileUri(path);
-        final FlutterOutlineListener listener = outlineListeners.remove(filePathOrUri);
+        final FlutterOutlineListener listener = outlineListeners.remove(path);
         if (listener != null) {
           getAnalysisServer().removeOutlineListener(FileUtil.toSystemDependentName(path), listener);
         }