Add notification on activity's start to pick the action bar's color.

I'm hooking onWindowFocusChanged because onPostResume isn't always
reliable.

As a bonus, the image gets refreshed when you pull down the notification bar…
or is that a bug?

Screenshots:
[1](http://i.imgur.com/ZVSwpgz.png)
[2](http://i.imgur.com/W81pu5X.png)
[3](http://i.imgur.com/UuLuzFF.png)