Keep playback speed for the player session

[anilbeesetti]

Changing the speed of one playlist video currently affects only that video; the next video restores its saved speed or the default. Initialize playback speed from the preference when the player session starts, then retain in-player changes across videos and queue replacements until the session ends. A new session starts from the preference again.

Remove per-video playback speed from media metadata, repository APIs, and stored video state. Migrate Room from version 12 to 13, discarding saved speeds while preserving the remaining playback settings. Add regression tests for session behavior and migration data preservation.

Fixes #2015

Validation

• ./gradlew assembleDebug test ktlintCheck --max-workers=4 passed; 296 unit tests, no failures, errors, or skips.
• ./gradlew :core:database:assembleDebugAndroidTest --max-workers=4 passed.
• On a disposable Android 9 / API 28 x86_64 emulator, adb shell am instrument -w dev.anilbeesetti.nextplayer.core.database.test/androidx.test.runner.AndroidJUnitRunner passed all 13 database tests, including the new migration.
• Device playback started at the configured 1.25× preference. Selecting 1.75× and advancing to the next video was attempted, but repeated System UI timeouts prevented reliable confirmation of navigation and fresh-session UI behavior. Those cases passed in the session regression tests. Gradle's connected-test device-property query also timed out, so database tests were run directly through adb.