android-4.9+: add CONFIG_NET_SCH_NETEM=y to recommended config

I've received multiple requests to enable the use of the 'netem'
packet scheduling qdisc.  I've also wanted to use it myself.

The amount of code is trivial (~1000 lines of code), but it enables
some pretty useful functionality - although mostly for debugging,
troubleshooting and testing.

One can use netem to introduce delays or packet loss and in general
reproduce a lower quality network.

Now that we've fixed iproute2 to enable support for more qdiscs,
it makes sense to also enable support for it in the kernel.

Bug: 124467469
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I0169514a997a77d044209a5b017d5f306fb3308e