PipeWire capturer: forget a restore token the portal did not renew

ScreenCastPortal only updates restore_token_ when the Start response
has a "restore_token". When the response has none, the token that was
passed in stays, and BaseCapturerPipeWire stores it in
RestoreTokenManager again. That token is already dead:
xdg-desktop-portal deletes a token as soon as it is used, so the next
capturer for that SourceId sends it for nothing.

Clear restore_token_ before reading the Start response, and always
record the result, so an empty token replaces the stale one. The entry
keeps its persist mode. A capturer that finds no token gets the portal
dialog, as it did when it sent the dead one.

Tested against a mock ScreenCast portal with a real PipeWire daemon.
With no token in the Start response the entry is now empty and a second
capturer sends none. A successful restore behaves as before.

Bug: webrtc:566241633
Change-Id: I95ce2fe2bc45a693fd45285ac7d6a2e384ae0422

Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/506700
Reviewed-by: Jan Grulich <jgrulich@redhat.com>
Commit-Queue: Shelley Vohr <shelley.vohr@gmail.com>
Reviewed-by: Johannes Kron <kron@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#48771}