fix getIfaceStats and getTotalStats bug

root cause: getIfaceStats and getTotalStats is directly reading
iface_stat_fmt or eBPF, not include tether stats.
solution: add tether stats to getIfaceStats and getTotalStats.

Bug: 120039819
Test case1:
1. tether offload is enabled on phone, enable MHS on phone.
2. Use test app to check getMobileRxBytes->getIfaceStats
3. Download 10M file on MHS client
4. Use test app to check getMobileRxBytes->getIfaceStats again
result: getMobileRxBytes increased around 10M

Test case2:
1. tether offload is disabled on phone, enable MHS on phone.
repeat above step 2~4
result: getMobileRxBytes increased around 10M

Following CTS cases passed
run cts -m CtsNetTestCases --test android.net.cts.TrafficStatsTest
run cts -m CtsUsageStatsTestCases --test android.app.usage.cts.NetworkUsageStatsTest

Change-Id: I3d94acb71c142ec38b750e58822881ff383341cc