feat(trailers): bring back in-app trailer playback

[silentbil]

Summary

Brings back ARVIO's in-app trailer player for the smaller trailer surfaces, and keeps YouTube's official player where trailers are shown large. In-app trailers have no YouTube logo, title bar or buttons and start noticeably faster; the official player gives HD.

Which player plays where

• In-app player: the expanding row card (classic and Netflix-style Home) and the classic Home hero background. On poster (vertical) rows, which have no expanding card, the hero plays the trailer instead. Any D-pad / OK / Back press stops a playing hero trailer on the classic Home.
• Official YouTube player: the Netflix-style Home billboard and the Details trailer. YouTube only serves 360p to the in-app player, which looks soft on these large surfaces; the official player plays 720p-1080p (measured on the emulator).
• Details on TV: the trailer now fills the screen. The bottom strip is gone; YouTube's own controls stay reachable (Up), OK pauses, Left/Right seek 10 s, Back closes and returns focus to the Trailer button. Mobile is unchanged.

Faster start

• As soon as Home knows a title's trailer key, it looks up the trailer's stream in the background, so in-app trailers usually start right after the focus delay.
• The trailer-key loading from main (cached and de-duplicated) is kept; the lookup is added to it.
• Trailer auto-play follows the profile's setting again. The existing trailer settings are unchanged.

Removed

• OpenYouTubeTrailer and its URL test (no longer used).

Testing

• Unit tests pass (./gradlew :app:testSideloadDebugUnitTest); the on-device tests compile.
• Tested on an Android TV emulator (1080p, API 30): billboard trailer in HD with the info on top, in-app row-card trailers, full-screen Details trailer with Back restoring focus, and Back / hold-Back on Home after merging the latest main.
• Not yet tested: classic Home (Netflix-style Home off) and mobile.