fs_config: fix fs_config_*_system targets

Content from fs_config designated for other partitions is leaking
into the system fs_config_* files.  Notably /vendor is showing up.

Rule expansion happens late, after all parsing.  This is the same in
make and kati.  build/make/tools/fs_config/Android.mk ends with
fs_config_generate_extra_partition_list :=, so by the time you get to
rule expansion the value is empty.

Added a PRIVATE_PARTITION_LIST variable to record the value of
fs_config_generate_extra_partition-list.

Test: inspect result of ${OUT}/product/*/system/etc/fs_config_files
Bug: 119677224
Bug: 119310326
Change-Id: I968b4936ab2c5d174f164c760820af13434654ca