Test for Apache HttpClient

Bug 4554251

This test launches multiple downloads at once to check that the
device can open up multiple sockets without OOMing. One test
tries to use wifi if available and the other tries to use
mobile by disconnecting wifi.

The CtsTestServer was modified to handle responses in separate
threads to test concurrent downloads.

Change-Id: Ifa1a11f409c69c1ae3ce621bf0542e0be56b50e0