Apple Inc.'s iPhone sales growth is outpacing that of its chief rival, Google Inc.'s Android mobile phones. However, the new numbers from Neilsen show that Google's Android operating system is still No. 1 among U.S. smartphone activations. Apple's iPhone has won the title of the fastest growing in market share. About 27 percent of the smartphones sold in the country from March to May used the Android system developed by Google (Nasdaq: GOOG).