Modify config to support app specific themes [1/4]

Summary of Changes
1. The configuration now supports each app to have their own theme.
Each theme can include icons, styles (ie overlays) and font.
2. The theme config is serialized into a json string and stored
in secure settings.  This entry consolidates what was previously 3 separate
keys (style, icon, font) into a single field.
3. 'CustomTheme' was renamed to 'ThemeConfiguration'.
Note: This commit is for the configuration. Additional work needs to be done to
the framework and UI to support app specific themes.

Change-Id: I226aaee85cfd1f57bb3b945566d69d3699a47395