Make the HttpResponseCache robust to file I/O errors.

If the filesystem fails, cache performance degrades. Ongoing
writes should not see any filesystem errors: don't punish the
writer who isn't benefitting from the cache. Ongoing reads may
see IOExceptions while streaming files. In practice this will
only happen if the directory disappears during use, as is the
case when a removable partition is removed.

Change-Id: Ibf4d51998d9beaba9f8c86c449fd5c97ca869cee
http://b/3180373
4 files changed