Default to CPU renering on Android to avoid GPU-rendering-specific driver woes
auto merge of #2148 : larsbergstrom/servo/android_201404c, r=metajack

On Android, our feeble GPU graphics stack is even less reliable than it is on Linux. To make it easier to bring Android devices online, this patch defaults to CPU rendering, which is much more stable and, frankly, faster.

r? @metajack