Bug 1051172: a willhide event hack so the camera can stop recording quicker when home button pressed

I've done some measurements, and launching an app from the homescreen is free as expected, and only returning to the homescreen introduces the extra slowness. It seems like it typically takes between 100 and 200ms to return to the homescreen.

Can we try calling this.sendWillHideNotification before switchApp, but in the same turn of the event loop? E.g., 

```
this.sendWillHideNotification(appCurrent);
this.switchAp(/* ... */);
```