rename: @btf.o -> @mainline.o

(in prep for these actually being only loaded via mainline netbpfload)

Test: TreeHugger
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I9cea40b3d7525aa3ef8fdaad291b27d7d179a1aa
diff --git a/Tethering/apex/Android.bp b/Tethering/apex/Android.bp
index 30bdf37..4bae221 100644
--- a/Tethering/apex/Android.bp
+++ b/Tethering/apex/Android.bp
@@ -103,9 +103,9 @@
         "dscpPolicy.o",
         "netd.o",
         "offload.o",
-        "offload@btf.o",
+        "offload@mainline.o",
         "test.o",
-        "test@btf.o",
+        "test@mainline.o",
     ],
     apps: [
         "ServiceConnectivityResources",