Fixed a crash when the currently applied theme is replaced by the PackageManager.

When a package is removed but still has active AssetManager's referring
to it, a request to retrieve the resource path (the APK) will fail, so
the assets cannot be removed.  The AssetManager's removeAssetPath API
was adjusted to now accept a cookie instead of searching for it by the
supplied asset path.  This allows us to still remove asset paths that
are no longer available.

Change-Id: I862c66e7515a1b05ca3e47f82e52cdf2f1d95684