Capture initial screenshot when starting frame timing trace (#55720)

Summary:
Pull Request resolved: #55720

Changelog: [internal]

When tracing starts, the frame metrics listener only fires when new frames are rendered. This means if no UI changes occur during a trace, no screenshots are captured. This also causes the initial state of the UI to be missing from the trace.

This change captures an initial screenshot immediately when `start()` is called, ensuring there's always at least one frame recorded at the beginning of a trace regardless of whether UI changes occur.

The implementation also refactors the frame emission logic into a shared `emitFrameTiming()` method to eliminate code duplication between the frame metrics listener and the initial capture.

Reviewed By: huntie

Differential Revision: D94223073

fbshipit-source-id: b84edae7caa15e24b437ef151e9a6c42916817d3

Author: rubennorte

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/devsupport/inspector/FrameTimingsObserver.kt

@@ -38,23 +38,7 @@ internal class FrameTimingsObserver(
       Window.OnFrameMetricsAvailableListener { _, frameMetrics, _dropCount ->
         val beginTimestamp = frameMetrics.getMetric(FrameMetrics.VSYNC_TIMESTAMP)
         val endTimestamp = beginTimestamp + frameMetrics.getMetric(FrameMetrics.TOTAL_DURATION)
-
-        val frameId = frameCounter++
-        val threadId = Process.myTid()
-
-        CoroutineScope(Dispatchers.Default).launch {
-          val screenshot = if (screenshotsEnabled) captureScreenshot() else null
-
-          onFrameTimingSequence(
-              FrameTimingSequence(
-                  frameId,
-                  threadId,
-                  beginTimestamp,
-                  endTimestamp,
-                  screenshot,
-              )
-          )
-        }
+        emitFrameTiming(beginTimestamp, endTimestamp)
       }
 
   private suspend fun captureScreenshot(): String? = suspendCoroutine { continuation ->
@@ -123,9 +107,33 @@ internal class FrameTimingsObserver(
       return
     }
 
+    // Capture initial screenshot to ensure there's always at least one frame
+    // recorded at the start of tracing, even if no UI changes occur
+    val timestamp = System.nanoTime()
+    emitFrameTiming(timestamp, timestamp)
+
     window.addOnFrameMetricsAvailableListener(frameMetricsListener, handler)
   }
 
+  private fun emitFrameTiming(beginTimestamp: Long, endTimestamp: Long) {
+    val frameId = frameCounter++
+    val threadId = Process.myTid()
+
+    CoroutineScope(Dispatchers.Default).launch {
+      val screenshot = if (screenshotsEnabled) captureScreenshot() else null
+
+      onFrameTimingSequence(
+          FrameTimingSequence(
+              frameId,
+              threadId,
+              beginTimestamp,
+              endTimestamp,
+              screenshot,
+          )
+      )
+    }
+  }
+
   fun stop() {
     if (Build.VERSION.SDK_INT < Build.VERSION_CODES.N) {
       return