i read here:
Onkyo | TX-SR309
Direct Digital Connection of iPod/iPhone via Front-Panel USB Port
The TX-SR309’s front-panel USB port lets you access MP3, WMA, AAC, and FLAC audio files stored on a USB flash drive or iPod/iPhone
does not state its compatible with Android, only iphone/thumb drive. this could be the issue.
EDIT: if you see the usb icon in the display on your phone, swipe down the notification drawer and see if there are other options avail.
in order words, pull down the notification drawer , tap on the usb icon if present, ad see if it gives you other options.