Refactor around apexKeysText singleton am: 2cf35e7bfd am: 2aa64a6139 am: 2e024d25d2

Original change: https://android-review.googlesource.com/c/platform/build/soong/+/2808896

Change-Id: Ibf0e674d8cf9c5d0106fb7577fa95b8debffb56d
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>