Fix Android PlatformMediaPlayer play/pause

[Copilot]

Three bugs in the Android PlatformMediaPlayer that together caused play/pause to not work correctly.

• Wrong ExoPlayer API: play()/pause() were toggling playWhenReady instead of calling exoPlayer.play()/exoPlayer.pause(), which don't fire the correct listener callbacks
• Missing state propagation: onPlaybackStateChanged doesn't re-fire when play/pause is called while already in STATE_READY, so PLAYING/PAUSED transitions are now driven from onIsPlayingChanged instead; the STATE_READY guard on the pause branch prevents PAUSED from being emitted during IDLE/ENDED teardown
• Duplicate polling loops: onIsPlayingChanged(true) fires on initial play, seek resume, and buffering recovery — each unconditional handler.post(updateProgressRunnable) spawned an extra self-rescheduling loop; fixed with removeCallbacks before post

override fun onIsPlayingChanged(isPlaying: Boolean) {
    if (isPlaying) {
        _state.value = PlaybackState.PLAYING
        handler.removeCallbacks(updateProgressRunnable) // cancel any in-flight tick
        handler.post(updateProgressRunnable)
    } else if (exoPlayer.playbackState == Player.STATE_READY) {
        _state.value = PlaybackState.PAUSED
    }
}

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