Add SignatureVerifier class to handle public keys from different sources am: 3bd07a5ec6

Original change: https://android-review.googlesource.com/c/platform/packages/modules/Connectivity/+/3375769

Change-Id: I692b47dcb253719d0cdad75c8fc761a006ed0fb7
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>