Move all the ImageWallpaper logic outside the main thread

In order to avoid any thread contention, all the ImageWallpaper logic should be moved out of the main thread. This CL moves the last things that were done on the main thread:
   - Call surface.lockCanvas and canvas.drawBitmap
   - Call reportEngineShown

Bug: 242969351
Test: atest ImageWallpaperTest
Test: treehugger
Change-Id: I2a4400c8eaf0d9b8a182e618655855188a15892b