Load *_compat_cil_file from system_ext as well

So we can extend platform policies with target specific compat rules.
This use case surface in the context of system only upgrade, when the
vendor policy cannot be updated, then the system_ext partition can
contain target specific compat policies.

Bug: 183362912
Test: Presubmit
Change-Id: Ic6436eb8a269f07f932331dedf7dbaa629538ade