[video_player_avfoundation] fix playback speed resetting

[misos1]

Calling play is the same as setting the rate to 1.0 (or to defaultRate depending on ios version, documentation in this first link is clearly not updated because it does not mention defaultRate):

https://developer.apple.com/documentation/avfoundation/avplayer/1386726-play?language=objc
https://developer.apple.com/documentation/avfoundation/avplayer/3929373-defaultrate?language=objc

The second link contains a note about not starting playback by setting the rate to 1.0. I assume this is because of the introduction of defaultRate (which can be different than 1.0) and not because play may do something more than just setting rate as that wording is explicit about setting rate to 1.0, it says nothing about any other value.

This is also why flutter/plugins#4331 did not work well. It was setting rate to 1.0 (through play) then immediately to the value set by setPlaybackSpeed. One of them or both caused change to playbackLikelyToKeepUp which triggered observeValueForKeyPath with playbackLikelyToKeepUpContext which in turn called again updatePlayingState where was rate again changed first to 1.0 then to another value and so on. In this PR the rate is changed only once and then to the same value, seems when rate is assigned but not really changed it does not trigger anything.

In issues below seekTo can trigger playbackLikelyToKeepUp change which will call updatePlayingState and reset rate to 1.0 through play. When the speed is set in dart before initialization then the dart side will set it right after calling play but even some time after initialization there is still a flood of events calling updatePlayingState and it is likely that some of them will call it after setPlaybackSpeed. Actually even when setPlaybackSpeed was not called on dart side it (dart side) will always change speed after play to 1.0 so it ignores this new defaultRate feature.

• fixes [video_player][IOS] SeekTo will affect the video using stream source flutter#71264
• fixes [video_player] Playback speed doesn't work when setting it before initialization  flutter#73643

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