Add GSettings schema for LUC handler

Add GSettings schema for LUC handler

The Last User Context will be stored/accessed using GSettings. This
requires:
- a schema of the data that will be stored.
- a modification to Makefile.am to validate and install the schema.
- modifications to the code to use GSettings.

This commit adds the schema and changes to Makefile.am. The
org.genivi.LUCHandler1.gschema.xml file is generated from
org.genivi.LUCHandler1.gschema.xml.in when ./configure is executed.
This is to allow the schema ID and base path to include the
BOOT_MANAGER_VERSION_API defined in configure.ac, in case we ever
decide to break the API or GSettings key structure.

The default value for the "last-user-context" key is an empty dictionary
for now.