DatagramChannelTest: use different value when testing SO_SNDBUF

1024 is below the minimum in some platforms (that is likely to
be platform in those platforms' kernels). Changing to 4096 as
to avoid hitting the minimum, since libcore is behaving correctly

(cherry picked from commit 2cee571dd14eb720c48220e055431ae637c86bda)

Bug: 27821554
Bug: 27893139
Change-Id: I96af16cd47a9b549635bd7a514a8133aac444b6f
1 file changed