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
Merged-In: I23e7ec140066979726d769cabc5f7057bb2167e6
(Cherry-picked from ag/14227609)
b1ff1b2139