Use PackedAdditionalOutputs when reading apk_set for canned_fs_config

When an app_set module is included in an APEX, we don't use its
`builtFile` which is the main APK, , but `PackedAdditionalOutputs()`
which is the zip file where all APK files (main + additional) files are
packed. We then unzip them into the corresponding directory inside the
APEX.

However, `buildFile` still has been used when constructing
canned_fs_config whose content should match with what are actually
included in the APEX. Fixing the bug by using
`PackedAdditionalOutputs()` instead.

Bug: 236299724
Test: m nothing (apex_test.go amended)
Test: follow https://buganizer.corp.google.com/issues/236299724#comment34
* mkdir for-236299724
* cd for-236299724
* repo init -u https://android.googlesource.com/platform/manifest -b
* master --partial-clone --clone-filter=blob:none
* repo sync -c -j8
* unzip -l ~/Downloads/mainline_t_2022_jun_t1004275.zip | cut -c31- | grep | xargs rm -f
* unzip ~/Downloads/mainline_t_2022_jun_t1004275.zip
* source build/envsetup.sh
* choosecombo 1 aosp_arm64 userdebug
* m com.android.extservices.gms

Change-Id: If9752c07748300dbb963568de4e879b041b0a206