Notification seek buttons, and resolve redirects to the real feed URL

Two things reported from the phone.

The playing notification had a previous button and no skip-forward. Media3's
default layout offers previous/next *track*, right for an album and wrong for
a podcast: with one episode loaded there is no next track, so a control was
dead and the useful ones absent. Skip-by-seconds are not default buttons --
they are custom session commands, and they have to be granted on connect or
the buttons appear and do nothing. Media3 ships numbered icons for 5, 10, 15
and 30 only, so back gets the 15 icon and forward gets a plain arrow rather
than one captioned 30; an icon stating the wrong number is worse than one
stating none.

The resolver returned the URL it was handed, not the one redirects landed on.
A share link from another app is a redirect to the real feed, so subscribing
through one stored that third party's URL as the feed: retire the shortener
and the subscription silently stops updating even though the publisher never
moved. It now resolves to the final URL, and discovered relative links resolve
against where the page actually was rather than where we asked.

Verified on device: an Apple Podcasts link for Common Sense resolves to
feeds.feedburner.com/dancarlin/commonsense and subscribes under the
publisher's own URL.

Also fixes lint errors this introduced -- Media3's session API is @UnstableApi
and the error result code belongs to SessionError, not SessionResult.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018AwnALZL6bD4VBtATDVv3B

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