ADD: [droid] backdoor to define XBMC_HOME and HOME on android

Principle: 
Define a ```/sdcard/xbmc_env.properties``` (hardcoded) with content similar to
```
xbmc.home=/sdcard/xbmc_home
xbmc.data=/sdcard/xbmc_data
```

Assets are cached to xbmc.home, and XBMC_HOME is set accordingly
HOME is set to xbmc.data (created id needed)

/cc @davilla