Fix build_prop module to use partition() instead of PartitionTag()

PartitionTag() returns the partition the module is installed in.
We don't want this because a system_ext_specific module could be
installed in the system partition if system_ext partition doesn't
exist.
Instead we want system_ext specific modules to always refer to
SystemExtPropFiles().

Bug: 359624775
Bug: 322090587
Test: build aosp_arm64 and verify system/system_ext/etc/build.prop
Change-Id: Ie67ec9c5e5a07e781515e0aafb42122f72447cf3