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: Id2a696c621015d46015d5de0e7aef987e9e01b54
Merged-In: Id2a696c621015d46015d5de0e7aef987e9e01b54