Schematize vold system properties

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

Bug: 117924132
Test: mma -j
Change-Id: Iaffcff950bfd2fbea20a1889ec6904955e099f9b
(cherry picked from commit 3d70e186647b6fcc5490f6178cf93f2fc257b616)