Have a new API to get private DNS mode

- Expose PRIVATE_DNS_MODE_OFF, PRIVATE_DNS_MODE_OPPORTUNISTIC and
PRIVATE_DNS_MODE_PROVIDER_HOSTNAME for external users.
- Since PRIVATE_DNS_DEFAULT_MODE_FALLBACK might be changed from
release to release, so it cannot be exposed as a system API.
Remove PRIVATE_DNS_DEFAULT_MODE_FALLBACK and have a new API -
getPrivateDnsMode() for users to get the private DNS mode instead.

Bug: 172183305
Test: atest FrameworksNetTests CtsNetTestCases
Test: atest com.android.cts.devicepolicy.DeviceOwnerTest#testPrivateDnsPolicy
Change-Id: I02a1e91b4eafb5f5df3eada1c07b99849a050c3c