Currently, it will always look in user 0 since it uses the DPM from
mContext, which will always be from user 0 as WPMS is in the system
server process.

Extend DPMI to provide the necessary external helper API. This is
preferable to just using createContextAsUser before getting the DPM
instance since it avoids a second binding.

Fixes: 144048540
Fixes: 172682826
Bug: 153995973
Bug: 174642338
Test: atest com.android.cts.devicepolicy.MixedManagedProfileOwnerTest#testSetWallpaper_disallowed
Change-Id: I52b71000fac31ff6725ddded58206f69b263ae33
(cherry picked from commit 5b36ee3f)