Fix test flake on IpConnectivityLogTest

The test does not wait for sending all log events done, just
check the total received event number after 200ms. If one of
thread is running slowly, the test would be flaky. Thus, use
the Thread#join() to ensure the log events sending out
completely.

Bug: 245005401
Test: atest CtsNetTestCases
Change-Id: I90ad6960453c4639b6429a2e36e35d2d53148131
1 file changed