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