Enable color functions for ThemePicker

- Add color section for ThemePicker.
- Define theme stub apk with rainbow colors.
- Video:
  https://drive.google.com/file/d/1s_cpn1OegbD1Lof2PxaUVV08BTwpzimp/view

Flag: No impact to existing product: just moves code to AOSP for reference
Bug: 218396282
Test: Push apks with paths below & test.
 /system_ext/priv-app/ThemePicker/ThemePicker.apk
 /product/app/ThemesStub/ThemesStub.apk
Change-Id: I25130573ffb37075b4b71c6b83b7b903b6421ce3
diff --git a/themes/AndroidManifest.xml b/themes/AndroidManifest.xml
new file mode 100755
index 0000000..68ead89
--- /dev/null
+++ b/themes/AndroidManifest.xml
@@ -0,0 +1,14 @@
+<?xml version="1.0" encoding="utf-8"?>
+<manifest xmlns:android="http://schemas.android.com/apk/res/android"
+    package="com.android.customization.themes"
+    android:versionCode="1">
+
+    <uses-sdk
+      android:minSdkVersion="31"
+      android:targetSdkVersion="31"/>
+
+    <application
+        android:hasCode="false"
+        android:label="@string/app_name"/>
+
+</manifest>