Create *_system_ext.mk for system_ext packages

The default packages for system_ext partition must be added to
*_system_ext.mk.
We have empty list for now, but will update the files soon.

Bug: 134359158
Bug: 139053989
Test: build and boot
Change-Id: I78c85c3a9ff0437906143c2d4982ecfb0e4d058d