Tested and working on Android 7.0:

    adb shell
    dpm set-active-admin --user current com.company.foo.bar.package/.the.Admin.Reciever

To find the admin receiver of an installed package, use the following to adb shell command and review the output:

    adb shell
    pm dump com.company.foo.bar.package | grep ' filter' | cut -d ' ' -f 12 | sort | uniq

To give a real world example, here is the command used to activate IBM's Maas360 Android client as a device admin:

    adb shell
    pm dump com.fiberlink.maas360.android.control | grep ' filter' | cut -d ' ' -f 12 | sort | uniq
    
    Output:
    …
    com.fiberlink.maas360.android.control/.receivers.GoogleCampaignReceiver
    com.fiberlink.maas360.android.control/.receivers.LocalEventReceiver
    com.fiberlink.maas360.android.control/.receivers.Maas360DeviceAdminReceiver  <-- This is the one I want
    com.fiberlink.maas360.android.control/.receivers.Maas360SecondaryDeviceAdminReceiver
    …


    Set Device Admin:    
    dpm set-active-admin --user current com.fiberlink.maas360.android.control/.receivers.Maas360DeviceAdminReceiver