WallpaperManagerCompat: correct build version check

WallpaperManagerCompatVOMR1 is targeting API 27 (O MR1), but the
api guard is checking, if we are running API 26 (O) or above.

Correct the check to prevent using unavailable Wallpaper APIs.

Change-Id: Iaf510bb79010e640db04adbbc2ee44de9abd6913
Signed-off-by: Alexander Martinz <amartinz@shiftphones.com>