Remove fs.readFile from some plugins

Summary:
1. Remove fs.readFile from some plugins
2. Add "importFile" to FlipperLib and RenderHost

See D32492149 for context

Followups:

1. Decapitate Stella's sendToPhone
2. Decapitate crash reporter
3. Figure out how to approach navigation
4. Figure out how to approach FileSelector

Reviewed By: mweststrate

Differential Revision: D32496775

fbshipit-source-id: e150aa56a2c2c1eb12a4c03c801f76cd76485a9d

desktop/flipper-plugin/src/__tests__/api.node.tsx

@@ -97,6 +97,7 @@ test('Correct top level API exposed', () => {
       "ElementSearchResultSet",
       "ElementsInspectorElement",
       "ElementsInspectorProps",
+      "FileDescriptor",
       "FlipperLib",
       "HighlightManager",
       "Idler",