Register APKCERTS_FILE on PACKAGES variable for prebuilt_apex

This is currently done for `soong_android_app_set.mk` but not for other
prebuilts (e.g. prebuilt_apex). In preparation for limiting apkcerts.txt
entries to the list of installed apps, register this variable.

Since a single apk can be in multiple prebuilt apexes, this CL also adds
a `sort -u` on the final output to prevent duplicates.

Bug: 399788149
Test: With RELEASE_APKCERTS_INSTALL_ONLY=true reapplied
(https://r.android.com/3552561), previously failed signing test now
passes
https://android-build.corp.google.com/builds/abtd/run/L03300030010481704

Test: presubmits
Change-Id: Ibe4f252317a8dbd25b735643f2c9142c75bd922b
2 files changed
tree: 9743623811e371e82462973a08de04210295e31a
  1. backported_fixes/
  2. ci/
  3. common/
  4. core/
  5. packaging/
  6. target/
  7. teams/
  8. tests/
  9. tools/
  10. .gitignore
  11. Android.bp
  12. banchanHelp.sh
  13. buildspec.mk.default
  14. Changes.md
  15. CleanSpec.mk
  16. Deprecation.md
  17. envsetup.sh
  18. help.sh
  19. navbar.md
  20. OWNERS
  21. PREUPLOAD.cfg
  22. rbesetup.sh
  23. README.md
  24. shell_utils.sh
  25. tapasHelp.sh
  26. Usage.txt
README.md

Android Make Build System

This is the Makefile-based portion of the Android Build System.

For documentation on how to run a build, see Usage.txt

For a list of behavioral changes useful for Android.mk writers see Changes.md

For an outdated reference on Android.mk files, see build-system.html. Our Android.mk files look similar, but are entirely different from the Android.mk files used by the NDK build system. When searching for documentation elsewhere, ensure that it is for the platform build system -- most are not.

This Makefile-based system is in the process of being replaced with Soong, a new build system written in Go. During the transition, all of these makefiles are read by Kati, and generate a ninja file instead of being executed directly. That's combined with a ninja file read by Soong so that the build graph of the two systems can be combined and run as one.