Improved the warning icon: Higher resolution, better size.

Not much to say here. The icon looked really unsharp on my new HTC m8, so I added a higher resolution version. Also added a little border, just like all other icons in android have. Changed the name as well because "sms" didn't really fit any more.

Old icon:
![old](https://cloud.githubusercontent.com/assets/1475672/3263811/b1fa050a-f276-11e3-9f13-608a0deb24f7.png)

New icon:
![new](https://cloud.githubusercontent.com/assets/1475672/3263810/b1f779a2-f276-11e3-9683-113cae70763b.png)