DO NOT MERGE: Accurately emulate historic Downloads ACTION_VIEW behavior.

Add write permision to VIEW intent (don't regress bahavior).
Add (already known) mimetype when creating intent.

Test: Download a CSV file to Downloads *root*.
    Install Sheets from play store.
    Navigate to Settings > Storage > Explore > (device name) > Download
    Tap CSV file.
    It should open.

NOTE: This is a (manual) cherrypick from master. Manual part necessary
as DocumentsUI has moved to a new repo in master.
https://googleplex-android.googlesource.com/platform/frameworks/base/+/a4e765f16e3c0031343af55df65380df44672825

Bug: 31245151
Change-Id: Ic64c655d33182e559f9645c1e173b2fe4b601d6c