android: writeLog perf improvements

[snowp]

Adds two performance optimizations that should help the inline overhead of logging on Android:

• Pass null instead of empty arrays when there are no fields/matching fields. This allows us to skip the JNI calls for empty arrays
• Manually manage the JNI local references instead of using a local frame, which seems to perform better due to not having to create lots of local frames during field conversion

---

• CHANGELOG.md's "Unreleased" section has been updated, if applicable.

[github-actions]

Size Comparison Report (x86_64)

Metric	APK (KB)	SO (KB)
Baseline	3370	1244
Current	3370	1244
Difference	0	0

APK size unchanged. SO size unchanged.

[github-actions]

Android Benchmark Results

Device: PR: Emulator - Android SDK built for x86_64 (API 31) | main: Emulator - Android SDK built for x86_64 (API 31)

Note: Benchmarks run on emulator. Results may vary between runs and may differ from physical devices.

Allocations

Test	PR	main	Δ
logHttpNetworkLog50FieldsAndHeadersAndFieldProviders	915	916	-0.1%
logNotMatched10Fields	17	18	-5.6%
logNotMatched5000Fields	46	45	+2.2%
logNotMatched500Fields	30	46	-34.8%
logNotMatched5Fields	11	14	-21.4%
logNotMatchedNoFields	1	1	0.0%
trackSpansWithFields	135	135	0.0%
trackSpansWithoutFields	55	56	-1.8%
webViewBridgeBridgeReady	61	62	-1.6%
webViewBridgeCustomLog	66	66	0.0%
webViewBridgeInvalidJson	38	38	0.0%
webViewBridgeWebVitalCLS	115	115	0.0%

Timing

Test	PR	main	Δ
logHttpNetworkLog50FieldsAndHeadersAndFieldProviders	235.00 us	234.09 us	+0.4%
logNotMatched10Fields	5.75 us	5.77 us	-0.4%
logNotMatched5000Fields	2.32 ms	2.25 ms	+3.0%
logNotMatched500Fields	352.78 us	238.56 us	+47.9%
logNotMatched5Fields	3.18 us	3.20 us	-0.6%
logNotMatchedNoFields	361 ns	446 ns	-19.2%
trackSpansWithFields	487.27 us	481.08 us	+1.3%
trackSpansWithoutFields	7.67 us	7.63 us	+0.5%
webViewBridgeBridgeReady	7.92 us	7.87 us	+0.5%
webViewBridgeCustomLog	8.62 us	8.57 us	+0.6%
webViewBridgeInvalidJson	83.37 us	139.79 us	-40.4%
webViewBridgeWebVitalCLS	15.88 us	15.57 us	+2.0%