This is a first scoop of implementing support for custom themeapk tag (see...

This is a first scoop of implementing support for custom themeapk tag (see Android Manifest). Here is a brief description of the changes this check in brings in: - PackageParser recognizes a themeapk tag and produces ThemeInfo object if needed; - getInstalledThemes methods (PackageManager and PackageManagerService) return a list of installed theme packages; - PackageInfo object includes ThemeInfo object from now on.