Delete outdated bitmaps when setting a live wallpaper

Also, refactor (very slightly) with a "clearWallpaperBitmap" method.
This is a prerequisite to make Backup & Restore work well.
This will make sure getWallpaperFile() always return null for live
wallpapers, which is what we expect.

Flag: lockscreen-lwp
Bug: 283091821
Test: atest WallpaperManagerTest
Change-Id: I3a5b226f866d3662946e0229fc4120ea7fa0fb1b