Inhibitors of β-Site Amyloid Precursor Protein Cleaving Enzyme (BACE1): Identification of (S)-7-(2-Fluoropyridin-3-yl)-3-((3-methyloxetan-3-yl)ethynyl)-5′H-spiro[chromeno[2,3-b]pyridine-5,4′-oxazol]-2′-amine (AMG-8718)