fix VoldExploitTest

The exception java.security.SocketException doesn't exist.
It should be java.net.SocketException.

SELinux blocks the creation of new netlink messages. Because
of the mistaken exception class, we try to throw an exception
which doesn't exist. This leads to the following error:

  art     : art/runtime/check_jni.cc:65] JNI DETECTED ERROR IN APPLICATION: c == null
  art     : art/runtime/check_jni.cc:65]     in call to ThrowNew
  art     : art/runtime/check_jni.cc:65]     from void android.security.cts.NetlinkSocket.create_native(java.io.FileDescriptor)

On Nexus devices, we don't see this error, because neither
of the following files exist:

  * /etc/vold.fstab
  * /system/etc/vold.fstab

However, on some OEM devices, these files continue to exist, even
though they've been removed from Android's source tree.
See https://source.android.com/devices/tech/storage/config.html

Addresses a CTS test failure on our partner's devices.

Bug: 18204494
Change-Id: I747e7fead05c526cd557bb4e34605aa4ddebcba0