AAPT2: Fix issue where @null was wrongly encoded

@null must be encoded as TYPE_REFERENCE with a value of
0. TYPE_NULL is used by the runtime as a placeholder when resolving
style attributes. If we set a style attribute to TYPE_NULL,
the runtime will throw. The runtime will convert a TYPE_REFERENCE
with value 0 to a proper null value.

Change-Id: Id983ca7e1fbee3124dddafe32f1b5741b824225b