Schematize vold system properties

Properties accessed across partitions are now schematized and will
become APIs to make explicit interfaces among partitions.

Bug: 117924132
Test: m -j
Change-Id: I8e04f643197b6c8a60cc38c6979e41c5de3469f5
(cherry picked from commit 0cf5fc1433ef113d33d40b62cfca9ce97de12a4b)