Introduce "BoxShadowDrawable" for Android box shadows

Summary:
This change adds a drawable, when when drawn on the bounds of a border-box sized view, will draw a spec compliant box-shadow outside the box. This is reliant on Android `RenderNode` and `RenderEffect` APIs provided by API 31.

Inset box shadows can also be added using a similar method, but this is not done yet.

The code which manages this is in flux, but the underlying drawable should be good. Will add some tests once it's more wired up.

Changelog: [Internal]

Differential Revision: D55561465

Author: NickGerleman

packages/react-native/ReactAndroid/api/ReactAndroid.api

@@ -5395,6 +5395,14 @@ public abstract interface class com/facebook/react/uimanager/debug/NotThreadSafe
 	public abstract fun onViewHierarchyUpdateFinished ()V
 }
 
+public final class com/facebook/react/uimanager/drawable/BoxShadowDrawable : com/facebook/react/uimanager/drawable/DecorationDrawable {
+	public fun <init> (Landroid/content/Context;Lcom/facebook/react/uimanager/drawable/CSSBackgroundDrawable;IFFFF)V
+	public fun draw (Landroid/graphics/Canvas;)V
+	public fun getOpacity ()I
+	public fun setAlpha (I)V
+	public fun setColorFilter (Landroid/graphics/ColorFilter;)V
+}
+
 public class com/facebook/react/uimanager/drawable/CSSBackgroundDrawable : android/graphics/drawable/Drawable {
 	public fun <init> (Landroid/content/Context;)V
 	public fun borderBoxPath ()Landroid/graphics/Path;

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/uimanager/drawable/BoxShadowDrawable.kt

@@ -0,0 +1,92 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+package com.facebook.react.uimanager.drawable
+
+import android.content.Context
+import android.graphics.Canvas
+import android.graphics.ColorFilter
+import android.graphics.Rect
+import android.graphics.RenderEffect
+import android.graphics.RenderNode
+import androidx.annotation.RequiresApi
+import com.facebook.react.uimanager.FilterHelper
+import com.facebook.react.uimanager.PixelUtil
+import kotlin.math.roundToInt
+
+// "the resulting shadow must approximate {...} a Gaussian blur with a standard deviation equal
+// to half the blur radius"
+// https://www.w3.org/TR/css-backgrounds-3/#shadow-blur
+private const val SHADOW_RADIUS_SIGMA_SCALE = 0.5f
+
+/** Draws an outer-box shadow https://www.w3.org/TR/css-backgrounds-3/#shadow-shape */
+@RequiresApi(31)
+public class BoxShadowDrawable(
+    context: Context,
+    private val background: CSSBackgroundDrawable,
+    private val shadowColor: Int,
+    private val offsetX: Float,
+    private val offsetY: Float,
+    private val blur: Float,
+    private val spread: Float,
+) : DecorationDrawable() {
+
+  private val shadowShapeDrawable = CSSBackgroundDrawable(context).apply { color = shadowColor }
+
+  private val renderNode =
+      RenderNode("box-shadow").apply {
+        setClipToBounds(false)
+        setRenderEffect(createBlurEffect())
+      }
+
+  override fun draw(canvas: Canvas) {
+    val spreadPx = PixelUtil.toPixelFromDIP(spread).roundToInt().coerceAtLeast(0)
+    val boundsWithSpread = Rect(bounds).apply { inset(-spreadPx, -spreadPx) }
+
+    with(shadowShapeDrawable) {
+      setBounds(0, 0, boundsWithSpread.width(), boundsWithSpread.height())
+      setRadius(background.fullBorderRadius)
+      for (location in CSSBackgroundDrawable.BorderRadiusLocation.values()) {
+        setRadius(background.getBorderRadius(location), location.ordinal)
+      }
+    }
+
+    with(renderNode) {
+      val offsetBounds =
+          Rect(boundsWithSpread).apply {
+            offset(
+                PixelUtil.toPixelFromDIP(offsetX).roundToInt(),
+                PixelUtil.toPixelFromDIP(offsetY).roundToInt())
+          }
+
+      setPosition(offsetBounds)
+      discardDisplayList()
+      beginRecording().let { nodeCanvas -> shadowShapeDrawable.draw(nodeCanvas) }
+      endRecording()
+    }
+
+    with(canvas) {
+      clipOutPath(background.borderBoxPath())
+      drawRenderNode(renderNode)
+    }
+  }
+
+  override fun setAlpha(alpha: Int) {
+    renderNode.alpha = alpha / 255f
+  }
+
+  override fun setColorFilter(colorFilter: ColorFilter?) {
+    val chainedEffect = colorFilter?.let { RenderEffect.createColorFilterEffect(it) }
+    renderNode.setRenderEffect(createBlurEffect(chainedEffect))
+  }
+
+  private fun createBlurEffect(chainedEffect: RenderEffect? = null): RenderEffect? =
+      if (blur <= 0f) chainedEffect
+      else FilterHelper.createBlurEffect(blur * SHADOW_RADIUS_SIGMA_SCALE, chainedEffect)
+
+  override fun getOpacity(): Int = (renderNode.alpha * 255).roundToInt()
+}