Address DPM Private DNS API Review

Changes:
* Split setGlobalPrivateDNS to setGlobalPrivateDNSModeOpportunistic
  and setGlobalPrivateDNSModeSpecifiedHost as the original method had a
  potentially unused parameter.
* Annotated return value.
* Annotated with @WorkerThread the method that shouldn't be called on
  the main thread.

Bug: 124301971
Test: atest com.android.cts.devicepolicy.DeviceOwnerTest#testPrivateDnsPolicy
Change-Id: Iace4de642bcf02ad6ee3a02804643890a5017d89