Move group_system_ext and passwd_system_ext to base_system_ext.mk

As we have base_system_ext.mk for the packages in system_ext
partition, move the group and passwd files for system_ext partition
to base_system_ext.mk.

Bug: 141658360
Test: mainline checks pass
Change-Id: Iff50482b90ff9877a58c54c8149014e8c103a3c6