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: 129375805
Test: manual
Exempted-From-Owner-Approval: storage refactoring
Change-Id: I89b08591523153a80374bd444b2855ecfc04936b