Warn when exposing file:// Uris beyond a process.

Check for file:// Uris inside Intents, ClipData, Notifications and
RemoteViews when StrictMode option is enabled.

Also introduces Intent.prepareToLeaveProcess() to uniformly handle
Intents about to leave an app process.

Bug: 8529070
Change-Id: I8efb43877cbc5f21eb029fc6492b3ee1415059ef