Do not use hidden SystemProperties.set

ConnectivityService is going to become a mainline module, and
it will not able to use hidden method anymore. Thus, use
alternative new sysprop as API to control the tcp init rwnd
value.

Bug: 170917042
Test: adb shell getprop net.tcp_def_init_rwnd and check if
      value is set correctly
Test: atest FrameworksNetTests
Change-Id: If9e99c88de50b6829721b0dfacc430a3b53c7728