Create the DiscoveryExecutor Class

This is a no-op refactor. Move the DiscoveryExecutor from
MdnsDiscoveryManager to a standalone class for subsequent
changes.

Bug: 366373064
Test: atest FrameworksNetTests NsdManagerTest
Change-Id: Idbfbbb423ddfe78a37db1b189aef945401b0a1e9
4 files changed