Revert DiskBasedCache. (#366)

The current implementation is not thread-safe, and making it thread-safe while
retaining an Asynchronous interface and allowing parallel reads would introduce
significant complexity. It is also limited to Android O+ which limits its value
at the moment.

For now, we can rely on either:

- AsyncRequestQueue's fallback support for regular Cache, which uses the blocking
  executor to perform reads. This is functional, but does make heavier use of the
  blocking executor than is ideal, and also does not currently permit parallel
  reads, although this could be added later as an improvement to DiskBasedCache.

- Cronet's own HTTP cache (although at the moment, we call disableCache(); we can
  make this configurable).

See #181