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After trying multiple methods in which to select the download location
(including a full-fledged folder selector, surfacing only the
cannonical directories like Music or Pictures), we decided on giving the
user only the option to select between internal and external
directories (ie. Downloads from internal storage vs. SD card). This CL
implements the user interface for this.

Some of the structural changes include:
- Implementing Spinners for preferences and the dialog, using styles
that are generic but match with the different context.
- Consolidating DownloadDirectoryList functionality into
DownloadDirectoryAdapter.
- Modifying DownloadDirectoryAdapter to be an ArrayAdapter and using it
as the adapter for the Spinners, keeping things uniform.

Screenshots are as follows:

Downloads preferences:
https://drive.google.com/open?id=1CNJG8e4M9Q3Seps5QyA6Sx45bOUewO0y
https://drive.google.com/open?id=1UnCCIGwdYeh5iOV1Cb2S4X9hjS-YV5s8

Download location dialog:
https://drive.google.com/open?id=1szZlaMiqGyWD6RoVU1B8fSoDqOjLl-KH
https://drive.google.com/open?id=1pKAATJMwaWoetHxGoiW4CqdqLiXj3kPx

Bug: 792775
Change-Id: Ie4aabed98f900d260298d98fe9dc172438bbb719
Reviewed-on: https://chromium-review.googlesource.com/959384Reviewed-by: Theresa <twellington@chromium.org>
Reviewed-by: David Trainor <dtrainor@chromium.org>
Reviewed-by: Shakti Sahu <shaktisahu@chromium.org>
Commit-Queue: Joy Ming <jming@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543120}