Various theme picker fixes.

- Add Previous button to custom flow
- Seed custom flow with the currently applied bundled theme
- Use add drawable icon instead of text
- Add padding between checkbox and "Add current wallpaper"
- Show ripple background on "Tap to edit"

Fixes: 130763331
Fixes: 130753131
Fixes: 130761402
Fixes: 130759176
Bug: 130759508
Test: visual
Change-Id: Ia82344befe8ac45489646d840453a71e31c83eae