Don't attempt to enable rate-limiting before T.

Rate-limiting cannot work because the BPF program is in the
mainline version of netd.c, which is placed into net_shared
and thus cannot run pre-T.

Disable it entirely to ensure no impact on S.

Test: atest ConnectivityCoverageTests:com.android.server.ConnectivityServiceTest on AOSP
Test: atest ConnectivityCoverageTests:com.android.server.ConnectivityServiceTest on S
Change-Id: I47521a100f8287ecdece25e810db8f3cade46778
2 files changed