Fix flaky testTcpBufferReset

If EVENT_NETWORK_INFO_CHANGED was handled after waitForIdle()
method done, this test will be failed due to
INetd#setTcpRWmemorySize() was not executed yet. Instead of
waiting for idle state, it's better to use NetworkCallback to
ensure events completely on ConnectivityService.

Bug: 140206578
Test: atest ConnectivityServiceTest#testTcpBufferReset\
      --generate-new-metrics 500

Change-Id: Iad77415db38038603881d9ebe0e37586b549a45c