Dexpreopt install path fix for system_ext apex sscp jars

The partition of these jars were hardcoded to `system`. With
https://r.android.com/3288083, these files will be installed in the same
partition as the parent apex. Kati built partitions got the correct
installation paths, but soong built partition paths of these files
evaluate to
/system_ext/system_ext/framework/oat/... at ToT

Test: no diff in kati built file_list.txt for system and system_ext
Test: Added com.android.compos locally to autogenerated system_ext
filesystem, and ran diff_test
Bug: 372487849

Change-Id: Ib486343b267fa2628fe65565b8e6b378730e3e81