Bug 1732343 - Part 2: Migrate all uses of base::FileDescriptor to UniqueFileHandle, r=handyman

This is useful for the following parts, as UniqueFileHandle is a cross-platform
type which can also be used to support transferring HANDLEs between processes.

This change requires fairly sweeping changes to existing callsites, which
previously did not require owning access to the handle types when transferring.
For the most part these changes were straightforward, but manual.

Differential Revision: https://phabricator.services.mozilla.com/D126564