Move bitmap for lock screen wallpaper into gfx

Bug: 145008953
Test: set lock screen wallpaper and check that it shows up on lock
screen.
Test: check app summary of `dumpsys meminfo` that Native Heap doesn't
increase by ~20 MB. I'm seeing the increase in EGL mtrack.

Change-Id: I1229d3f10f050a0530bf63668e9b1724551aad41