service: Introduce a global Settings object

Introduced a global Settings object that will store all runtime properties that
would be associated with a config file, Android system properties, etc. Added a
mechanism to parse command-line options which can be used to pass paths to
configuration files, file path for UNIX domain socket based IPC mechanism, and
any other property that is dynamic in nature. This will help us remove hardcoded
paths, strings, and other such values in the future.

Bug: 22532366
Change-Id: I8e790363ed31d44369f7991a8ea7132d1cace70b