Use system API to obtain IDnsResolver IBinder

Connectivity service module is using a ServiceManager @hide API
to get IDnsResolver IBinder. Replace it with
DnsResolverServiceManager system API for incoming mainline.

Bug: 170598012
Test: atest FrameworksNetTests
Test: Manually connect and disconnect Wi-Fi/Cellular network
Change-Id: I457dccd44e5adc8c4d9b3c2853323228db3a0d89