Swimming in first-person view no longer hides your hands

Relates to MC-128242 and MC-133414. From the introduction of the swimming ability in snapshot 18w07a to snapshot 18w14b, swimming in first-person view consistently hid your hands (but not your held items). However, in snapshot 18w15a, when the swimming mechanics were updated, a bug caused this feature to malfunction. The visibility of your hands would only update correctly when switching back to first-person view from third-person view. When this bug was addressed in snapshot 18w19a, instead of restoring the previous functionality, the feature stopped working entirely, likely due to an oversight. Notably, this feature was present in the Bedrock Edition of Minecraft, but instead of instantly hiding your hands, there was a small animation. This animation was also added to the Legacy Console Edition after it had already stopped working in Bedrock, as mentioned in MCPE-161216. The removal of this feature may be intentional, but that decision ultimately lies with Mojang. Therefore, this ticket should remain open until Mojang reaches a conclusion.

Steps to reproduce

1. Hold nothing or a map in your main hand
2. You can also hold a map in your offhand if you wish
3. Swim underwater

Observed: Hands remain visible, unlike in previous versions.
Expected: Hands should be hidden, as consistently observed from version 18w07a to 18w14b.