Add an IPC that writes to the tcp_{rmem, wmem}

The system server is controlling the tcp buffer now by writing to
/sys/kernel/ipv4/tcp_{rmem,wmem}_{min,def,max}. Those files are
basically the same as /proc/sys/net/ipv4/tcp_{rmem,wmem} except those
latter ones contain all three values in one file. Netd can directly write
to those files instead of depending on the android specific implementation.

Test: netd_integration_test
Bug: 118572798
Change-Id: I588b48be29ecf61fd5bbf94f97f63738be4eae25