Merge "Add support for missing public / private key files for apexes." am: babb1337

Original change: https://android-review.googlesource.com/c/platform/build/soong/+/2586953



Change-Id: I85b420c76881664ebca704887b04f92326df182c
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>