Fix flaky NsdManagerTest

Bad synchronization between the mock handler and the test assertions
were causing testDiscoverService() to sometime fails (1/50 ~ 1/100 repro
rate).

Bug: 32561414
Bug: 62918393
Test: runtest frameworks-net, NsdManager#testDiscoverService now passes
      1000 times without errors.
Change-Id: I0308cdf48fc6bdc2fc9b4e8f7d5241a1cf2ea443