Add tether BPF offload config to device config and resource

The tether bpf offload can be enabled by resource config and
device config. The device config has higher priority and it
could override this config which is set by resource config.

Bug: 149997301
Test: -build, flash, boot
      -atest TetheringConfigurationTest
Original-Change: https://android-review.googlesource.com/1276007

Use device option to control BPF offload features

If BPF offload device config is not enabled:
- Does not add/remove offload forwarding rules through disabling IP
  neighbor monitor.
- Does not apply the RA MTU reduction.

Bug: 149997301
Test: atest IpServerTest
Original-Change: https://android-review.googlesource.com/1284578
Merged-In: I2d6f80f0229f580c4b16243a064e889a6c37f77a
Change-Id: I2d6f80f0229f580c4b16243a064e889a6c37f77a