Handle android_app_set in Soong built apkcerts.txt

android_app_set modules contain presigned apks, and uses `extract_apks`
to create an entry for each split apk in `LOCAL_APK_CERTS`, which then
gets concatenated to apkcerts.txt when built using Make.

This logic was previously not handled correctly in the new Soong
implementation, causing build errors in partner workspaces.

Bug: 396131789
Test: In partner-workspace, built the soong generated apkcerts.txt file
Change-Id: I7dd0dcd6c992660d5a7e7d484b5d83dc06e051f3