Theme engine: Support launcher based icon packs

Before this patch, icon packs written for Trebuchet/Nova/Apex could not be used
with the new theme engine as the formats were different. Particularly the icon package IDs are 7F,
the same ID as as regular apps. Furthermore the location of resources in these icon packs was different
from what the theme engine expects. This patch adds support for these icon packs by overriding
the pkg id.

AssetManager now can override package ids without requiring aapt to recompile the APK.
Note: XML references will still use the original id compiled with AAPT, so it is only useful for resources
which do not use xml drawables (such as icon packs).

PackageInfo now has a new member identify "legacy" (3rd party icon packs) written
for launcher's like Trebuchet, Nova, and Apex.

Change-Id: Icd12c097e156c9626df0772abeb50e00f7b61f64