Fix: install flattened apex on system_ext

This build rule is specific to platform APEXes.
For non-platform APEXes, MakeAsSystemExt() is not applied.

This fixes the cases of "soc_specific: true" apexes which fails to
build.

Bug: 139053989
Test: m nothing (soong tests)
Change-Id: I98d0257499647ab41cdaa62a3671d89addbdf833
Exempt-From-Owner-Approval: got +1 before rebasing