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
diff --git a/res/layout/activity_custom_theme.xml b/res/layout/activity_custom_theme.xml
index 82b7887..b2aaa73 100644
--- a/res/layout/activity_custom_theme.xml
+++ b/res/layout/activity_custom_theme.xml
@@ -35,6 +35,13 @@
         android:layout_height="@dimen/custom_theme_nav_height"
         android:layout_marginHorizontal="12dp">
         <Button
+            android:id="@+id/previous_button"
+            style="@style/ActionSecondaryButton"
+            android:layout_width="wrap_content"
+            android:layout_height="wrap_content"
+            android:layout_gravity="start|center_vertical"
+            android:text="@string/custom_theme_previous"/>
+        <Button
             android:id="@+id/next_button"
             style="@style/ActionPrimaryButton"
             android:layout_width="wrap_content"