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

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

Change-Id: I3d85055e8093586b6f43382014a2cc2a642bdf7c
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>