fix: prevent crash on exit from double unbindService when Stop Music on Task Clear is enabled

[mvanhorn]

🚀 What does this PR do?

• 🐛 Fixes a bug (Issue [Bug]: Stop Task on Clear #647)
• ✨ Adds a new feature
• 🎨 UI/Design update
• ⚡ Performance improvement
• 🧹 Code refactoring

With "Stop Music on Task Clear" enabled, exiting the app crashes with java.lang.IllegalArgumentException: Service not registered from MainActivity.onDestroy -> unbindService. The same crash is reported in #771 and #737.

The cause is a double unbindService on the same serviceConnection. onStop() already unbinds unconditionally, and onDestroy() unbinds again inside the StopMusicOnTaskClearKey && isPlaying && isFinishing branch. Since onStop() runs before onDestroy(), the connection is already gone and the second call throws.

This adds a serviceBound flag set when bindService succeeds in onStart() and guards both unbindService call sites so each unbind matches exactly one bind. stopService() and clearing playerConnection are unchanged.

📸 Screenshots / Recordings

n/a (crash fix, no UI change)

🛠️ Checklist

• My code follows the project's style guidelines.
• I traced the fix against the Activity lifecycle scenarios in [Bug]: Stop Task on Clear #647, [Bug] : App crashes when exiting app #771 and [Bug]: Crash after exiting with back gesture #737 (normal exit, swipe-from-recents with Stop on Task Clear, and rapid foreground/background cycling). A device/emulator build was not available in my environment, so a maintainer check on hardware would be appreciated.
• I have verified that no new warnings/errors are introduced.
• I have updated the documentation (no docs change needed for this fix).

📝 Additional Notes

Change is local to MainActivity.kt and is a small bind-state guard.

Fixes #647