further bpf_existence_test code simplifications/clarifications am: e9e7741872 am: a757505f31 am: 6485b462f8 am: 2fb918b9ab am: f2a921a1e4

Original change: https://android-review.googlesource.com/c/platform/packages/modules/Connectivity/+/2156256

Change-Id: I86149ea9a3a5a4e10fcc7fb9d621942b19289394
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>