Add Abstract Class for RealtimeScheduler

Because some APIs in RealtimeScheduler are higher than the min
SDK version for the standalone MDNS build, an abstract class is
required to allow the standalone MDNS build to pass.

Bug: 366373064
Test: atest FrameworksNetTests NsdManagerTest
      m service-connectivity-mdns-standalone-build-test
Change-Id: I0b3ac7b4f8dfceeb3dd81e6a5f368b5d57927c41
8 files changed