Apply suggested edits to the file-download API

* Rename Manager -> Session
* Add Handler for callbacks in download flow
* Separate out registering a state callback into another method
* Rename dispose -> close, implement AutoClosable
* No more thrown MbmsExceptions
* Add sanity check for temp file root
* Remove the getNames method, replace with getNameForLocale

Bug: 30981736
Test: testapps
Change-Id: I1a2054d79c934bc4929464de4a644b6960db47e9