Fixing admins not being removed from DpmMockContext

We were not calling setActiveAdmin before calling setDeviceOwner or
setProfileOwner in these tests. They were passing because the admins
were not being properly removed in the first place. The admin is
actually removed from the admin list inside onReceive of the receiver
passed in sendOrderedBroadcastForUser, which was not overridden to call
onReceive in DpmMockContext.

Bug: 30726135
Change-Id: I08cab5821228738196b229a8f885f7488aaafa34