android: add phonegap 3.1 tweak

Start from phonegap 3.1,
- DroidGap class is no longer used. It's replaced by `CordovaActivity`.
- `init()` method is called explicitly in the `onCreate()` method. To 
make Moodstocks phonegap plugin work properly, this line need to be 
replaced by `this.init()`.

A short description is added too.
android: set web view background to transparent

This solves the random launch flickering problem.