Fix getAttributeSetSourceResId to AttributeSet created from null.

Xml.asAttributeSet with a null returns non XmlBlock.Parser type
object.
This fixes the existing test that broke due to my changes to
LayoutInflater.

Test: atest CtsViewTestCases:LayoutInflaterTest
Change-Id: Ib10e07ef3814c5570a6d2bd334481f2e26bb3d3e