Fix activatePlugin measurements not completing

Summary:
`activatePlugin` events where not recorded correctly for Sandy plugins. Although the starting measuerments is fired the `connections` reducer, the completing event fires from `PluginContainer`. Since this is done as part of a ref-update (!!), see [here](https://www.internalfb.com/intern/diffusion/FBS/browse/master/xplat/sonar/desktop/app/src/PluginContainer.tsx?commit=65a625ea9941&lines=155), and the ref to the corresponding element is not set for Sandy plugins, the event was never marked as completed.
Fixed this by making it part of the `activate` life-cycle event of Sandy plugins.

Reviewed By: passy

Differential Revision: D25421537

fbshipit-source-id: 5cbfeb91cc12e4520fa271bab24034094d7ddb39

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

@@ -35,7 +35,7 @@ export interface Logger {
   debug(data: any, category: string): void;
 }
 
-export const _LoggerContext = createContext<Logger>({
+export const stubLogger: Logger = {
   track() {},
   trackTimeSince() {},
   info() {
@@ -54,7 +54,9 @@ export const _LoggerContext = createContext<Logger>({
     // eslint-disable-next-line
     console.debug.apply(console, arguments as any);
   },
-});
+};
+
+export const _LoggerContext = createContext<Logger>(stubLogger);
 
 /**
  * Provides the default logger that can be used for console logging,