Call netd to update the tcp buffer size

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 so we no longer need to depend on these android specific
files.

Test: netd_integration_test
Bug: 118572798
Change-Id: I588b48be29ecf61fd5bbf94f97f63738be4eae25