Emit C++ TraceSection markers through ATrace on Android

[tomekzaw]

Summary:

TraceSection (ReactCommon/cxxreact/TraceSection.h) is the C++ tracing primitive used throughout the renderer: ShadowTree::commit, UIManager::createNode, the Yoga measure callbacks, FabricMountingManager::executeMount and others. On Android it only does something when built with WITH_PERFETTO or WITH_FBSYSTRACE, and the OSS build sets neither, so it compiles to DummyTraceSection.

Current behavior. Perfetto and systrace captures of OSS Android apps show only the Java/Kotlin sections. The C++ half of the renderer (commit, layout, text measurement, building mount instructions) is invisible.

With this change. A new Android branch calls the NDK's ATrace_beginSection and ATrace_endSection:

• The functions are resolved once with dlsym, so no target gains a link dependency on libandroid.
• Each section checks ATrace_isEnabled() first, so when nobody is tracing it costs one call.
• WITH_PERFETTO and WITH_FBSYSTRACE keep precedence.
• Section arguments (key/value pairs) are ignored for now.

Why. Without this, profiling React Native on Android with standard tools can't show where commit and layout time goes. In a capture of RNTester mounting a 100-post feed, this branch shows 7084 UIManager::createNode sections and 1632 Yoga measure-callback sections, among others; on main none of them appear.

Changelog:

[ANDROID] [ADDED] - C++ TraceSection markers now appear in Perfetto and systrace captures

Test Plan:

• RNTester assembleDebug and assembleRelease (-PreactNativeArchitectures=arm64-v8a) and yarn format-check-cpp pass.
• Recorded Perfetto traces (atrace_apps: "com.facebook.react.uiapp") of the same RNTester scenario on main and on this branch. The C++ sections appear only on this branch.
• Overhead with tracing off. Emulator (Pixel 10 Pro AVD, API 37), release builds, current main vs. this branch, 6 runs, medians:

scenario	total CPU, main	total CPU, this PR	change
mount 100-post feed	128.8 ms	130.0 ms	+1%
mount 1000 labels	117.7 ms	117.1 ms	−1%

Update phases were within emulator noise, with mixed signs.

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