Add android_splash_screen (#903)

Adds a Flutter app designed for devices running at least Android 12 that exemplifies how to implement an animated splash screen with the SplashScreen API.

android_splash_screen/android/build.gradle

@@ -0,0 +1,29 @@
+buildscript {
+    ext.kotlin_version = '1.5.31'
+    repositories {
+        google()
+        jcenter()
+    }
+
+    dependencies {
+        classpath 'com.android.tools.build:gradle:4.1.0'
+        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
+    }
+}
+
+allprojects {
+    repositories {
+        google()
+        jcenter()
+    }
+}
+
+rootProject.buildDir = '../build'
+subprojects {
+    project.buildDir = "${rootProject.buildDir}/${project.name}"
+    project.evaluationDependsOn(':app')
+}
+
+task clean(type: Delete) {
+    delete rootProject.buildDir
+}