Add default lock wallpaper capability and API

If present, the system property "ro.config.lock_wallpaper" provides a
filesystem path to a decodeable image file to use as the system's
out-of-the-box lock wallpaper imagery.  In the absence of this
system property, or if the indicated file is absent or unreadable,
then the new framework resource
com.android.internal.R.drawable.default_lock_wallpaper is consulted to
locate a usable asset.  This mechanism parallels the existing one for
the default system wallpaper.

By default there is no specific lock wallpaper asset; the resource is
defined to be @null in the standard config.xml file.  A product that
wants to define such a factory-default lock-only wallpaper image
will provide the asset as part of its framework resource overlay.

Bug 27828056

Change-Id: Iebf3706222370d0a0a4baf88d71a59ead07a25c7