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	3427	1292
Current	3427	1292
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	924	923	+0.1%
logNotMatched5000Fields	10	10	0.0%
logNotMatchedNoFields	1	1	0.0%
trackSpansWithFields	140	140	0.0%
trackSpansWithoutFields	55	54	+1.9%
webViewBridgeBridgeReady	60	60	0.0%
webViewBridgeCustomLog	61	65	-6.2%
webViewBridgeInvalidJson	38	38	0.0%
webViewBridgeWebVitalCLS	178	176	+1.1%

Timing

Test	PR	main	Δ
logHttpNetworkLog50FieldsAndHeadersAndFieldProviders	247.82 us	241.00 us	+2.8%
logNotMatched5000Fields	2.49 ms	2.46 ms	+1.5%
logNotMatchedNoFields	381 ns	459 ns	-16.9%
trackSpansWithFields	487.04 us	476.97 us	+2.1%
trackSpansWithoutFields	7.86 us	7.75 us	+1.4%
webViewBridgeBridgeReady	8.21 us	8.05 us	+2.0%
webViewBridgeCustomLog	12.49 us	8.83 us	+41.4%
webViewBridgeInvalidJson	100.80 us	165.91 us	-39.2%
webViewBridgeWebVitalCLS	21.73 us	20.50 us	+6.0%