Remove DefaultState in NsdService

The code handling service registration, discovery, and resolution
in DefaultState is never executed because these messages are
always handled in EnabledState. Therefore, the active code in
DefaultState should be moved to EnabledState and DefaultState
should be removed.

Bug: 307209858
Test: atest FrameworksNetTests CtsNetTestCases
Change-Id: I38a255ddf72148c7ae13d3079dbd3c03f800b9c2