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/plugin/PluginBase.tsx

@@ -172,6 +172,9 @@ export abstract class BasePluginInstance {
       this.activated = true;
       this.flipperLib.enableMenuEntries(this.menuEntries);
       this.events.emit('activate');
+      this.flipperLib.logger.trackTimeSince(
+        `activePlugin-${this.definition.id}`,
+      );
     }
   }