Themes: Support applying components from multiple themes [1/2]

The current implementation assumes the components being passed
into the ThemeService belong to the same theme.  This patch uses
a Map instead of a List to pass in a mapping of components to
package names, allowing us to apply several components from
different themes.

Requesting a theme change now takes a Map<String, String> which
maps components to the theme package name they will be applied
from.
Example:
    Map<String, String> componentMap = new HashMap<String,String>();
    componentMap.add(ThemesColumns.MODIFIES_ICONS, "com.mytheme.blue");
    componentMap.add(ThemesColumns.MODIFIES_OVERLAYS, "com.mytheme.red");

    ThemeManager tm = (ThemeManager) contex.getSystemService(Context.THEME_SERVICE);
    tm.requestThemeChange(componentMap);

Change-Id: I13fa17eabecb824884fe522d5dc1822a4d7c55f4