Themes: Only process themes that are applied during boot

Currently, the PackageManagerService will process all themes and
ensure their resources are compiled and available to the system.
This is highly inefficient and can bog the system down when booting.

This patch will only process the themes that are currently applied
durring boot and lets the ThemeService take care of queuing up the
others for processing.   This reduces boot time considerably,
especially when the user has many themes installed.

Change-Id: I52168a1751ad3bfc458048dd1558ad71cc16317a