Move netd.c to Tethering module

This is a clean move. The content of netd.c is not changed. The object
name is still netd.o. But the module name is renamed to netd.o_mainline
to avoid name collision in sc-mainline-prod branch.

Modified Android.bp according to the file location. The sub_dir is
newly specified. The object file will be compiled to:
- apex/com.android.tethering/etc/bpf/net_shared/netd.o

The extracted programs and maps will be in:
- sys/fs/bpf/net_shared/

The netd.o will not be loaded in pre-T because the bpfloader before T
does not load objects from paths other than:
- apex/com.android.tethering/etc/bpf/
- /system/etc/bpf/

Bug: 202086915
Test: cd system/netd; atest
Test: atest TrafficStatsTest NetworkUsageStatsTest
Change-Id: I5281c851341f9258a37d8aad6da4196c06342940
diff --git a/Tethering/apex/Android.bp b/Tethering/apex/Android.bp
index 9f67b39..9363c29 100644
--- a/Tethering/apex/Android.bp
+++ b/Tethering/apex/Android.bp
@@ -63,6 +63,7 @@
     ],
     canned_fs_config: "canned_fs_config",
     bpfs: [
+        "netd.o_mainline",
         "offload.o",
         "test.o",
     ],