Avoid redundant wallpaper loading/drawing

Switching users causes wallpapers to get recreated 3 times. Other operations
like startup and rotation cause similar redundant load/draw operations. This change
tracks the various attributes that tell us when we really need to reload
and redraw, causing only one of these expensive operation per one of these
switches.

Issue #7334664 Wallpaper draws several times when switching users

Change-Id: Ic3072ef3a7eaf622d8632e87e34f50999f716c39