Desks: Default DesktopWallpaperActivity focusability to false

This is a follow up to Idc7ac2c374bee72bb54bd7b3a5daca915262af25.

When the activity is created for the first time, setting the focus to
true resulted in the wallpaper being touchable behind a desktop app even
though it should've been made unfocusable by a call to
onTopResumedActivityChanged. To work around this lifecycle bug, set the
focus to |false| by default. It should in theory result in a reversed
bug if entering desktop through an empty desk, but
onTopResumedActivityChanged does call back in that case.

Flag: com.android.window.flags.enable_multiple_desktops_backend
Fix: 416700931
Test: reboot, open fullscreen task, enter desktop using handle menu -
verify the wallpaper isn't touchable
Test: reboot, open fullscreen task, swipe up to overview, create empty
desk, enter empty desk - verify there's no ANR on keyboard events

Change-Id: I039abd5051b03ab17f3468434cf77c364ced403f