Fix symlinks from APEX to partitions

Previously, the symlink optimization for APEXes assumed that the target
of the symlinks are in the system partition. The assumption however
doesn't hold always because the file that was added to the APEX might be
with system_ext_specific: true or vendor: true.

Bug: 265598720
Test: m nothing
Change-Id: Ieb9a6769320c0ec697a88c0cae977e7d65288362