Fix AnimationPlayer seeking for Discrete keys

[TokageItLab]

• Fixes AnimationPlayer does not get the Discrete key correctly when seeking if there is an precision error #85483
• Fixes Animations created in AnimationPlayer in 4.2 don't update frames while scrubbing the timeline #85632

Internal seeking exists to determine the start of playback in AnimationNodes on the AnimationTree. So in the AnimationPlayer, this should always be true, since AnimationPlayer doesn't have internal seeking.

Also the playback process has changed with the AnimationMixer implementation, and the timing of the animation time advance has changed, so the fix in #80708 should not need to be considered.

Tested with #80640 and confirm it is working correctly.

[fire]

I don't fully understand this, but the pr is a single line and written by the primary animation team developer. So the pull request looks good to me.

[LakshayaG73]

I can confirm that this fixes #85129 and doesn't seem to introduce any new issues in my project.

[akien-mga]

Thanks!

[YuriSizov]

Cherry-picked for 4.2.1.