Remove the need of accessing handler in NSS unit test

Currently, to wait for handler becomes idle, specific message
is used and the test would wait for condition variable to be
open when the message is processed.

However, this is already done in the HandlerUtils. Thus,
there is no need to post such message manually in the handler.

Test: atest FrameworksNetTests
Bug: 150664039

Change-Id: Iab32b2dbab01634ca159dcb90fc9f929d1fed1a2