init.rc: prep tcp_default_init_rwnd, set the default to 60

The kernel's default is between 4~20.

Prepare for javaland to modify the value at runtime.

It can be done via
  setprop sys.sysctl.tcp_def_init_rwnd  <value>


Bug: 12020135
Change-Id: Id34194b085206fd02e316401c0fbbb9eb52522d2