Fix NPE with missing DPMI

DevicePolicyManagerInternal is not always available (when device admin
is not an enabled feature), so treat it as nullable in
WallpaperManagerService.

Does not need to be cherry-picked since cherry-pick CL
I103c45ea61814e476eea197d357ef0ce75edefdc already accounts for this.

Fixes: 174642338
Bug: 144048540
Bug: 172682826
Bug: 153995973
Change-Id: Ib7ad0d6d1038ad4dd3fec7c617ef3c23c911293d