Add CancellationSignal support to file operations.

Since ContentProvider file operations can end up doing substantial
network I/O before returning the file, allow clients to cancel their
file requests with CancellationSignal.

Ideally this would only be needed for openFile(), but ContentResolver
heavily relies on openAssetFile() and openTypedAssetFile() for common
cases.

Also improve documentation to mention reliable ParcelFileDescriptors
and encourage developers to move away from "rw" combination modes,
since they restrict provider flexibility.  Mention more about places
where pipes or socket pairs could be returned.

Improve DocumentsContract documentation.

Bug: 10329944
Change-Id: I49b2825ea433eb051624c4da3b77612fe3ffc99c