DO NOT MERGE: Fix temporary black after setting a new image wallpaper

Sometimes see temporary black wallpaper after setting a new image
wallpaper because of:
- decode bitmap twice, results in long latency until drawing.
- the set wallpaper apis didn't sync with wallpaper rendering well.
Solutions:
- Only decode bitmap when necessary.
- Make set wallpaper apis wait for image wallpaper rendering finished.

Bug: 194080642
Test: see b/194080642#comment3
Test: atest WallpaperManagerServiceTests --iterations 50
Test: atest SystemUITests
Change-Id: I369eff5195571161cf286b3ec97175481b02eed7