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/test-utils/test-utils.tsx

@@ -380,6 +380,7 @@ export function createMockFlipperLib(options?: StartPluginOptions): FlipperLib {
     openLink: jest.fn(),
     showNotification: jest.fn(),
     exportFile: jest.fn(),
+    importFile: jest.fn(),
     paths: {
       appPath: process.cwd(),
       homePath: `/dev/null`,