perf: Faster WritableNativeMap.putString value conversion

[riteshshukla04]

Summary:

I found (while working on #58611) that WritableNativeMap::putString converted its value with fbjni toString(), which calls Java Object.toString() through JNI before running toStdString(). Since the value is already a jstring, I call toStdString() directly, which produces identical bytes with one fewer JNI call and local reference.

Value	Before	After	Delta
11 ASCII	0.205	0.191	-6.8%
100 ASCII	0.265	0.231	-12.8%
300 ASCII	0.451	0.418	-7.3%
1000 ASCII	0.984	0.966	-1.8%
100 CJK	0.325	0.298	-8.3%

Changelog:

[ANDROID] [CHANGED] - Skip a redundant JNI Object.toString() call in WritableNativeMap.putString.

Test Plan:

I measured putString at 0.614 → 0.474 µs per call for short strings (-23%) on a Galaxy M14, identical output on 21 edge-case strings.