Remove Preconditions, Protocol @hide usage

Remove hidden API usage of the Preconditions and Protocol classes in
NsdManager.

NsdManager does not share its handler with other components, so it does
not need to use specific message IDs from Protocol.
Preconditions can also be easily replaced with stable API alternatives.

Bug: 190249673
Test: atest FrameworksNetTests CtsNetTestCases:NsdManagerTest
Change-Id: Ia173f99b82c90df702291c3690f4a7b047e90daa