Remove support for file:// items.

We've messaged since the N release that file:// Uris are going away,
and we've been crashing those apps via StrictMode for many years.

The broader storage changes in Q mean it's finally a good time to say
we only handle content:// items.

Bug: 123212933
Test: none
Change-Id: I69a791468c4bcf45b0022cf52264e78f94bfdeae