Address comments in ag/1514806.

* Isolate LinkedList and stack/root manipulation in DocumentStack
* Consolidate ProviderAccess into DocumentAccess
* Pass Uri as the the param to LoadDocStackTask
* Tidy up initLocation() in picker.Actionhandler
* Add a test case for initLocation with data in picking intents

Bug: 32156176
Change-Id: I3ca07342fc594e47fc003df146dcf982f152f56d