[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

apex/apex.go

@@ -1574,8 +1574,8 @@ func apexFileForRuntimeResourceOverlay(ctx android.BaseModuleContext, rro java.R
 	return af
 }
 
-func apexFileForBpfProgram(ctx android.BaseModuleContext, builtFile android.Path, bpfProgram bpf.BpfModule) apexFile {
-	dirInApex := filepath.Join("etc", "bpf")
+func apexFileForBpfProgram(ctx android.BaseModuleContext, builtFile android.Path, apex_sub_dir string, bpfProgram bpf.BpfModule) apexFile {
+	dirInApex := filepath.Join("etc", "bpf", apex_sub_dir)
 	return newApexFile(ctx, builtFile, builtFile.Base(), dirInApex, etc, bpfProgram)
 }
 
@@ -1784,8 +1784,9 @@ func (a *apexBundle) GenerateAndroidBuildActions(ctx android.ModuleContext) {
 			case bpfTag:
 				if bpfProgram, ok := child.(bpf.BpfModule); ok {
 					filesToCopy, _ := bpfProgram.OutputFiles("")
+					apex_sub_dir := bpfProgram.SubDir()
 					for _, bpfFile := range filesToCopy {
-						filesInfo = append(filesInfo, apexFileForBpfProgram(ctx, bpfFile, bpfProgram))
+						filesInfo = append(filesInfo, apexFileForBpfProgram(ctx, bpfFile, apex_sub_dir, bpfProgram))
 					}
 				} else {
 					ctx.PropertyErrorf("bpfs", "%q is not a bpf module", depName)