release: prepare TypeType Android 1.9.0

[Priveetee]

Summary

• Prepare the Android 1.9.0 stable release from the tested 1.9.0-beta.3 line.
• Set the application version to 1.9.0 with version code 10900.
• Add the supplied 1.9.0 release notes.

Release scope

• Improve YouTube live playback, SABR fallback and prewarming, and BiliBili/NicoNico media playback.
• Add previous and next player controls, animated loading placeholders, and search service switching.
• Improve subscription notifications with UnifiedPush encryption keys and periodic local checks.
• Move the Android app from beta to stable while retaining Android 6.0 through Android 17 support.

Validation

• 823 unit tests: 0 failures, 0 errors, 1 skipped.
• :app:lintDebug, :app:assembleDebug, and :app:assembleRelease passed.
• git diff --check passed.
• Manual notification check: a synthetic encrypted UnifiedPush payload was delivered through Sunup and appeared as an active TypeType notification. This verifies push transport and client display, not new-video feed discovery.

Rollback

Restore the previous v1.8.0 stable release from GitHub or the stable F-Droid channel. Stable and beta use the same application identity, so switching channels updates the existing installation.