Move ImageWallpaper tasks to its own thread

Previously, ImageWallpaper performed its long-running tasks (in particular the bitmap loading) with the @Background executor.

Since the bitmap loading can be very long (a few seconds) for some
wallpapers, and since the background executor is single-threaded and is
used for other UI tasks, we now use a dedicated executor to avoid
contention.

This CL also adds a provider to a Delayable LongRunning executor in SysUIConcurrencyModule, to be able to use the long running executor for the delayed bitmap unloading.

Test: manual
Test: atest GtsWallpaperTestCases
Test: atest WallpaperTest
Bug: 265018814
Bug: 265060422
Change-Id: I0f3320dee67c1ddd5f6294c92c4f5887237a9ed8