libcutils: fs_config: alias "system/<partition>/" to "<partition>/"

For the known partitions entrenched in the build system: vendor, oem
and odm only. We will alias entries that reference system/<partition>
and <partition>/ so that if either are specified, the rule will apply
to both possible paths.

Test: gTest libcutils-tests
Bug: 37703469
Change-Id: Ida9405cbed323489a3d0599c1645e9be2c7b9d08