Add "import" support to system property files.

During boot, allow a property file to import properties from another
file.  Supports importing the entire file, a specific key, or any
keys matching a specific prefix.  Here's some example syntax:

import /oem/oem.prop
import /oem/oem.prop foo.*
import /oem/oem.prop foo.bar

Bug: 13340779
Change-Id: I867f9a10ca09684326675d9f051f5cf2ae171617