Stop reading buttons back out of the pointer payload under construction

[christophpurrer]

Summary:
WARNING: Generated by Autopilot (alpha) — review carefully, verify the underlying claim before accepting.
Agent: React Native Agent (Bugs) | Trajectory: https://www.internalfb.com/intern/devai/devmate/inspector/8e9d02d0-ad79-40df-b2c0-8dbc0f0c39d1/ | SC job: https://www.internalfb.com/intern/sandcastle/instance/54043196151461660/

---

PointerEvent.createW3CPointerEvent built its payload map, then read one field
straight back out of that same half-built map to compute another field:

pointerEvent.putInt("buttons", getButtons(_eventName, pointerType, buttonState))
...
getPressure(pointerEvent.getInt("buttons"), _eventName)

Reading from a WritableNativeMap while still writing to it is not free and not
safe:

• ReadableNativeMap.getInt materialises the whole map across JNI
  (importKeys + importValues) and memoises the result in keysStorage /
  localMapStorage. Every subsequent put* on the same instance then leaves
  those caches stale, so a later Kotlin-side read of the map (hasKey,
  toHashMap) does not see pressure, tangentialPressure,
  hitPathForEventListener or the modifier keys.
• It happens on the pointer-event hot path, once per pointer index per dispatch,
  purely to recover a value the caller already has in hand.

Keep the value in a local and pass it to getPressure directly. The payload is
byte-for-byte identical: getInt returns exactly the Int that putInt stored,
so getPressure receives the same argument as before.

Changelog:
[Internal]

Differential Revision: D118471070

[meta-codesync]

@christophpurrer has exported this pull request. If you are a Meta employee, you can view the originating Diff in D118471070.