Enhance time-spent tracking

Summary:
Previously, at 1-minute intervals, if the flipper window was focused on, it would report the currently active plugin.
We'd sum all those "ping" events and that would approximate the number of full minutes spent in total across all users.
It's quite coarse grained, if you're focused on the window for 30 seconds, there's a 50% change your ping will get used.
While being reasonable across many users, it doesn't allow analysis like how many plugins do people typically use in a session, because we probably won't see all the plugins they use.

New approach, for every minute flipper is open, report the focused and unfocused time spent in each plugin, as well as the total across all plugins.
This should give us the previous data but with much more precision.

Should be especially helpful for plugins with low numbers of users, you typically interact with emulators while using a plugin, so it's not continually in focus, so you miss a lot of usage events.

enhance_bladerunner

Reviewed By: nikoant

Differential Revision: D19392796

fbshipit-source-id: af9244e993edff9b381144ca587c3a77fdf8c98a

static/index.js

@@ -97,7 +97,7 @@ let filePath = argv.file;
 
 // tracking
 setInterval(() => {
-  if (win && win.isFocused()) {
+  if (win) {
     win.webContents.send('trackUsage');
   }
 }, 60 * 1000);