libcutils: reimplement system property functions with libbase.

This makes the system property functions available on the host too, and
ensures a consistent view of system properties between libcutils and
libbase, if you have code running on the host that uses both.

Bug: http://b/151789258
Test: treehugger
Change-Id: Ie524a77d0c392d7b23e9d12becbb1bf53c81eac6