Fix the side effect of `adb shell dpm set-device-owner`

When there was already an active admin, the command created a fresh
active admin and replaced the existing one, which cleared some
necessary flags that were previously set in the admin.
This CL fixes the unwnated side effect by not overwriting the existing
admin.

Bug: 282239828

Test: adb shell dpm set-device-owner --user 0 <component>
      adb shell dumpsys device_policy

Change-Id: Ib424dcbcec4e3699beb5aa46079424dc9d87d275