Allow access to the underlying DiskLruCache.

This is needed to implement public android APIs
android.net.HttpResponseCache.{flush,size,maxSize,close}
as well android.net.HttpResponseCache.install.

install needs to inspect the underlying DiskLruCache because
it promises not to install a new cache if the file directory and
size are the same as the old cache.
Merge pull request #208 from narayank/master

Allow access to the underlying DiskLruCache.