Port from snappy branch: userdata -> writable

[Barry Warsaw]

The snappy branch switched the cache_partition from /android/cache to /writable/cache/

This needs to be supported in si 3.0, but it may be as simple as a configuration file given that we now support a config.d directory and multiple config files. At least, add a test for this.

[Barry Warsaw (barry)]

For si 3.0, snappy will just need to write an additional configuration file which contains:

[updater]
cache_partition: /writable/cache

This should be in a file with a higher index prefix, such as 10_snappy.ini

No changes are needed in si.