DO NOT MERGE: Stabilize testConcurrentServerSocketCloseReliablyThrows.

As commented inside the test code, iterating this test too many times
will make it flaky by piling up sockets in TIME_WAIT state and eventually
failing. Reducing the iteration number mitigates the issue.

The iteration number is already reduced in the master branch for
speeding up the test: http://r.android.com/258101/. The current CL
extracts from the master CL only the part relevant to the flaky test.

Bug: 63265871
Test: CtsLibcoreTestCases (on affected device)
Change-Id: I9b121e180835e58a6ed419f78d0619418b1030fa