Added support for setting string resources in generated strings.xml

The following adds support for setting resources to add in strings.xml - it works very similar to setting the --meta-data tag. Here is the syntax:

./build.py ... --resource [KEY]=[VALUE]

where KEY is the string name / key and VALUE is the value for that specific KEY. Here is a sample usage:

./build.py ... --resource app_id=616xxxxxxxxxx

The generated strings.xml file now contains:
...
<string name="app_id">616xxxxxxxxxx</string>