adb: decrease the number of fds used in fd_count test.

Default limit is 1024 fds, which means 512 socketpairs will hit the
limit.

Bug: http://b/28386497
Change-Id: Ib36404c050e4be3db51378e250f300f7ec5aab13
diff --git a/sysdeps_test.cpp b/sysdeps_test.cpp
index f0c334e..fde344a 100644
--- a/sysdeps_test.cpp
+++ b/sysdeps_test.cpp
@@ -218,7 +218,7 @@
 
 TEST_F(sysdeps_poll, fd_count) {
     // https://code.google.com/p/android/issues/detail?id=12141
-    static constexpr int num_sockets = 512;
+    static constexpr int num_sockets = 256;
     std::vector<int> sockets;
     std::vector<adb_pollfd> pfds;
     sockets.resize(num_sockets * 2);