Cupertino gallery app (#2715)

Resolves #2519 

Part of the larger effort to bring the samples/repo into a stable and
maintainable condition that _also_ remains useful. See: #2409

## Pre-launch Checklist

- [x] I read the [Flutter Style Guide] _recently_, and have followed its
advice.
- [x] I signed the [CLA].
- [x] I read the [Contributors Guide].
- [x] I have added sample code updates to the [changelog].
- [x] I updated/added relevant documentation (doc comments with `///`).

cupertino_gallery/android/build.gradle.kts

@@ -0,0 +1,24 @@
+allprojects {
+    repositories {
+        google()
+        mavenCentral()
+    }
+}
+
+val newBuildDir: Directory =
+    rootProject.layout.buildDirectory
+        .dir("../../build")
+        .get()
+rootProject.layout.buildDirectory.value(newBuildDir)
+
+subprojects {
+    val newSubprojectBuildDir: Directory = newBuildDir.dir(project.name)
+    project.layout.buildDirectory.value(newSubprojectBuildDir)
+}
+subprojects {
+    project.evaluationDependsOn(":app")
+}
+
+tasks.register<Delete>("clean") {
+    delete(rootProject.layout.buildDirectory)
+}