Fixed ESPRESSO-257, ESPRESSO-258.

ESPRESSO-257 was fixed by a trivial change to check the original theme
package name for an empty string (in addition to null).

ESPRESSO-258 required addition of new AssetManager values to track
whether the AssetManager should have themes attached, persisting this
value across configuration changes.  We should once again respect is the
isThemeable flag by keeping themed assets out of the AssetManager of
apps that don't ask for it.

CR: Ed Carrigan