Set up compose (2/3)

Set up gradle and soong build config to enable using Compose in
customization picker. Also add a flag for the compose refactor and use
it to gate a basic WIP composable for the color floating sheet.

Flag: com.android.wallpaper.compose_refactor_flag
Bug: 391918848
Bug: 391927276
Test: manually verified by building with gradle and soong
Test: manually verified by flipping flag and checking color picker
Change-Id: Ie7038116cc129b2578d7f478faa9f0c9f6a4dd19