[Downloads] Open media downloads internally
* Force the Custom Tab Intent to use the same package as Chrome
when opening the media viewer.
* Open media downloads inside of the internal media viewer when
opening the file in the notification or via the snackbar.
* Shuffle code around for creating media viewer Intents in the
hopes of consolidating them in places that makes sense.
BUG=
650448
Committed:
https://crrev.com/4b442abfbfcd93afdd00d7dd106e71bddbaf19ac
Cr-Commit-Position: refs/heads/master@{#422624}