ConnectivityService: add support to set TCP initial rwnd

The value for the TCP initial receive window comes from,
in order,

kernel
  /proc/sys/net/ipv4/tcp_default_init_rwnd

init.rc (via properties)
  net.tcp.default_init_rwnd

properties
  net.tcp.default_init_rwnd

gservices
  Settings.Global.TCP_DEFAULT_INIT_RWND

Bug: 12020135
Change-Id: I0e271be19472900fa9f3bab037d53383ec014a9e