Update PrivateDnsMode from StringDef to IntDef

Update PrivateDnsMode from StringDef to IntDef because IntDef is
the normal way of representing multiple choices in public API.
Also update other related files.

Bug: 185311744
Test: 1. make update-api
      2. atest FrameworksNetTests CtsNetTestCases CtsNetTestCasesLatestSdk
      3. atest FrameworksServicesTests:DevicePolicyManagerTest
Change-Id: I23e7ec140066979726d769cabc5f7057bb2167e6