[NETD-BPF#34] Add a tag for bpf to specify the install folder

Currently, the bpf module netd.o (source system/netd/bpf_progs/netd.c)
will be built to /system/etc/bpf/netd.o. In Android T, it will be moved
to mainline module com.android.tethering.

The expected behavior is:
- In T device, it uses the netd.o in mainline module.
- In pre-T devices, it uses the original netd, built from platform.

However, netd.o will be double loaded if the tethering module is
installed in Pre-T devices. Because:
1. bpf in apex is packed into /apex/MAINLINE_MODULE/etc/bpf/
2. bpf in platform is packed into /system/etc/bpf/
3. bpfloader in pre-T loads ANY bpf modules under
/apex/com.android.tethering/etc/bpf/ and /system/etc/bpf/.

We can't change the behavior of bpfloader in pre-T devices. We can't
delete the /system/etc/bpf/netd.o from pre-T devices. Both of them are
not mainline modules. So the mainlined netd.o needs to be packed into a
folder other than /apex/com.android.tethering/etc/bpf/ or
/system/etc/bpf/.

This commit adds a tag 'sub_dir' for bpf module. The installation path
of bpf modules will be:
- /system/etc/bpf/SUB_DIR/     (for platform code)
- /apex/MAINLINE_MODULE/etc/bpf/SUB_DIR/     (for mainline module)

Bug: 202086915
Test: add test in apex_test.go and build
Change-Id: Icc6619768ab006de9f86620a7df1bb2853eaba13