The type of ConnectivityManager#PRIVATE_DNS_MODE_OPPORTUNISTIC
has changed from String to int, but the String definition is still
needed to update to Settings.Global.PRIVATE_DNS_MODE, so the
simplest way is to define one locally.
Bug: 185311744
Test: atest CtsNetTestCases
Change-Id: Iafcd861714d8aca44cede658ed630f9d5afd5e59
Merged-In: Iafcd861714d8aca44cede658ed630f9d5afd5e59
(Cherry-picked from ag/14232792)