ANDROID: namespace'ify tcp_default_init_rwnd implementation

Not needed in 4.19+ since this Android specific sysctl was
not included in later kernels.

Test: via uml net tests with namespaces enabled
Bug: 149894399
Bug: 166680775
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I2ad1dbc977d40ee260bde23c6ed32f2706082660
(cherry picked with simple conflict resolution fixups from
 android-4.14-stable commit 08870bd1a24fc7f3ae4ff30bc7e64c09edd931d4,
 retested via:
   ARCH=um SUBARCH=x86_64 /aosp-tests/net/test/run_net_test.sh --builder all_tests.py
 with patch to make adb detection, ie. HasEstablishedTcpSessionOnPort() always true)
9 files changed