Provide hit and miss stats on the HTTP response cache.

Also capture FDs rather than InputStreams when a cache
entry is read. This permits the cache to return as many
streams as the caller requires.

Also fix bugs where we weren't properly releasing the
input streams from the responses.

Change-Id: I04807eab648864229f1e8734ad1fbb6a2d6bb51d
http://b/3180373
8 files changed