Summary:
Device plugins have an activate and deactivate hook, that reflects the plugin being selected in the UI. Added these same hooks to client plugins as well. In practice they are called at the same times as `onConnect` and `onDisconnect`, except for background plugins, which connect only once, so it is pretty useful to be still able to make the distinction.
Since there is now quite some common functionality between plugins and device plugins, will clean things a bit up in a next diff
[Interesting] as it explains the difference between the different lifecycle methods of plugins, and the impact of being a background plugin
LIfecycle summary:
1. app connects
2. for background plugins: connect them (`onConnect`)
3. user selects a plugin, triggers `onActivate`. will also trigger `onConnect` the plugin if it _isnt_ a bg plugin
4. user selects a different plugin, triggers `onDeactivate`. will also trigger `onDisconnect` if it isn't a bg plugin.
5. app is unloaded. Triggers `onDisconnect` for bg plugins. Triggers `onDestroy` for all plugins,
Reviewed By: jknoxville
Differential Revision: D22724791
fbshipit-source-id: 9fe2e666eb37fa2e0bd00fa61d78d2d4b1080137
b9c9e89b53