Add default settings for Android multilibrary projects

Some settings have been added to the .buckconfig file in order to handle Android multilibrary projects under Intellij and prevent to duplicate the way they are configured. This can useful when dealing with a project formerly managed by Gradle and the Android plugin.

Settings added : 
```python
[project]

# defines each library's Manifest relative path
default_android_manifest_path = /src/main/AndroidManifest.xml

 # defines each library's resource folder relative path
default_android_resource_path = /src/main/res

# defines each library's assets folder relative path
default_android_assets_path = /src/main/assets
```

These settings are then used by intellij.py to generate Intellij's iml files. If they are not defined, the script falls back on the default settings ('/res', '/assets', '/AndroidManifest.xml').