Add PRODUCT_SYSTEM_DEFAULT_PROPERTIES

PRODUCT_SYSTEM_DEFAULT_PROPERTIES will be used to define system default
properties which should be installed in system partition.

Bug: 64661857
Test: confirmed that ART default properties are stored in
/system/etc/prop.default when they were defined in
PRODUCT_SYSTEM_DEFAULT_PROPERTIES.

Change-Id: Ia08c25d0c5805381c6e3fe63dd1d171e8d195b90