Introduce pluginKey

Summary: Exposed the `pluginKey` to sandy plugins (which we will use later for storing table preferences per plugin). And a little moving code around because circular deps problem roared its ugly head again.

Reviewed By: nikoant

Differential Revision: D27009721

fbshipit-source-id: 7ad29e72ff8028c9daae270c4749b657bd8ff049

desktop/flipper-plugin/src/test-utils/test-utils.tsx

@@ -234,6 +234,7 @@ export function startPlugin<Module extends FlipperPluginModule<any>>(
     flipperUtils,
     definition,
     fakeFlipperClient,
+    `${fakeFlipperClient.id}#${definition.id}`,
     options?.initialState,
   );
 
@@ -313,6 +314,7 @@ export function startDevicePlugin<Module extends FlipperDevicePluginModule>(
     flipperLib,
     definition,
     testDevice,
+    `${testDevice.serial}#${definition.id}`,
     options?.initialState,
   );