Fix error when setting 3rd party LWP with no WallpaperColors

Ensure getLowResBitmap is called from a background thread.

Bug: 213189715
Test: manual
Change-Id: Ie8a17928997ad75e5a4a785e9aa5819380b37c2d
(cherry picked from commit a979207d20a7a438ef2c87c43784ecc00e8d29f6)
(cherry picked from commit 60024feda2ecc3c31220a8df2fd02c995d7cbabd)
Merged-In:Ie8a17928997ad75e5a4a785e9aa5819380b37c2d
1 file changed