[chrome.power] Added support for system requestKeepAwake on Android

Based on https://developer.android.com/training/scheduling/wakelock.html#cpu, I've added support for system `chrome.power.requestKeepAwake` on Android.

![image](https://cloud.githubusercontent.com/assets/634478/3720291/47ca5428-1652-11e4-89d7-d3c7bb7e6776.png)

I think general practice is to report these in JS land, since most cca devs don't do native dev or don't even have sdk installed to track logcat.  (Hence why Max originally had the check in js).

I'm going to land this as is, since I'm sure we get this wrong in very many places in our plugins, but figured I'd mention this goal if you're going to keep making awesome patches for us!