allow use of custom ImageRequests in ImageLoader

This seems like the smallest change that will allow developers to
supply their own ImageRequest class to the ImageLoader without
changing the API. There are several use cases for supplying
custom ImageRequests, including setting custom headers, adjusting
the priority of ImageRequests, and working around Issue #71928.

Change-Id: Ic47452b5f8cada2491a6513cb56f383261c73c3a
Merge "allow use of custom ImageRequests in ImageLoader"
Don't hold onto requests indefinitely.

Move the request into NetworkDispatcher's loop so that the previously
processed request's reference is not held until the next request
comes off the queue.
Merge "Don't hold onto requests indefinitely."
Merge branch 'master' of https://android.googlesource.com/platform/frameworks/volley
sync with official repo, update readme