Handle mock tags in android.nfc.Tag conversion to Parcel.

When generating a mock tag (after a NDEF exchange over LLCP), one of
the internal fields is set to null. This was causing NullPointerException
when being converted to a Parcel.

This is fixed by not including this field in the Parcel for mock tags.

Change-Id: I000e2faa54d71fd755ba7993e1e258743aad98fb