Remove AsyncChannel usage in NsdManager

Replace AsyncChannel with two AIDL interfaces: INsdManagerCallback and
INsdServiceConnector.

This will be helpful to move NsdManager into a module, avoiding the
dependency on the hidden AsyncChannel utility.

Bug: 190249673
Test: atest FrameworksNetTests CtsNetTestCases:NsdManagerTest
Change-Id: Id9e8feb95cbb5ba16f00893f6c3f37fced3358c1