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