Adjust custom theme flow preview icon

 - Change custom icon page thumbnail icon to widget icon, make it can
 load the drawable from Launcher package
 - Change preview icon(airplane mode) to battery icon
 Screenshot: https://screenshot.googleplex.com/9fp9tpW7hbTeA5s.png

Bug: 159376354
Test: Manually, make full rom with ag/12947502, install local WPPG and check visually
Change-Id: Iba98e8bed43ca0c7a393be42061beba78e5fed9d