Updated Android Min SDK Version

As discussed earlier, since CrossWalk supports 4.x Android devices, `android:minSdkVersion` should be 14.

TEST=
```bash
$ cca create helloWorld
$ cca prepare
$ more platforms/android/AndroidManifest.xml | grep minSdkVersion
```
BUG=#271

![image](https://cloud.githubusercontent.com/assets/634478/3916676/eb44aeda-237e-11e4-9d3f-2e7991f33f27.png)