Don't build multi-arch apks when --webview=system is used.
Don't build multi-arch apks when --webview=system is used.

We did this already when webview=system in manifest.mobile.json. This
fixes when --webview=system is passed on command-line

wudkj/mobile-chrome-apps,guozanhua/mobile-chrome-apps,guozanhua/mobile-chrome-apps,MobileChromeApps/mobile-chrome-apps,guozanhua/mobile-chrome-apps,guozanhua/mobile-chrome-apps,MobileChromeApps/mobile-chrome-apps,chirilo/mobile-chrome-apps,xiaoyanit/mobile-chrome-apps,xiaoyanit/mobile-chrome-apps,liqingzhu/mobile-chrome-apps,MobileChromeApps/mobile-chrome-apps,wudkj/mobile-chrome-apps,xiaoyanit/mobile-chrome-apps,chirilo/mobile-chrome-apps,liqingzhu/mobile-chrome-apps,wudkj/mobile-chrome-apps,MobileChromeApps/mobile-chrome-apps,liqingzhu/mobile-chrome-apps,liqingzhu/mobile-chrome-apps,xiaoyanit/mobile-chrome-apps,chirilo/mobile-chrome-apps,chirilo/mobile-chrome-apps,wudkj/mobile-chrome-apps