Adding dynamic_theme and gemini_tasks (#2287)

Adds two new samples that use the Google Generative AI SDK for Dart.
Both were created by @rodydavis. I'm merely the uploader so to speak.

## 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 updated/added relevant documentation (doc comments with `///`).
- [X] All existing and new tests are passing.

dynamic_theme/android/build.gradle

@@ -0,0 +1,18 @@
+allprojects {
+    repositories {
+        google()
+        mavenCentral()
+    }
+}
+
+rootProject.buildDir = '../build'
+subprojects {
+    project.buildDir = "${rootProject.buildDir}/${project.name}"
+}
+subprojects {
+    project.evaluationDependsOn(':app')
+}
+
+tasks.register("clean", Delete) {
+    delete rootProject.buildDir
+}