Lazy load mHandlerThread in WallpaperManagerService.

mHandlerThread was added in ag/24804347 to improve the duration and
the spikiness of the user switch, with a small memory cost.
But this memory regression was observed even when there is no
multiple users present or supported on the device. This CL fixes that
issue by lazy loading the mHandlerThread.

Bug: 323841616
Fixes: 321880416
Test: Manual control of Perfetto traces
Test: atest WallpaperManagerTest
Flag: android.multiuser.bind_wallpaper_service_on_its_own_thread_during_a_user_switch
Change-Id: I004a725abf50b282ada36a8cb734279f30284e64