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/test-utils/test-utils.tsx

@@ -36,6 +36,7 @@ import {
 } from '../plugin/DevicePlugin';
 import {BasePluginInstance} from '../plugin/PluginBase';
 import {FlipperLib} from '../plugin/FlipperLib';
+import {stubLogger} from '../utils/Logger';
 
 type Renderer = RenderResult<typeof queries>;
 
@@ -346,6 +347,7 @@ export function renderDevicePlugin<Module extends FlipperDevicePluginModule>(
 
 export function createMockFlipperLib(options?: StartPluginOptions): FlipperLib {
   return {
+    logger: stubLogger,
     enableMenuEntries: jest.fn(),
     createPaste: jest.fn(),
     GK(gk: string) {