[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)

Super awesome work!

Can you add an override of onReset() as well that releases the wake lock?