Let UsbDeviceManager choose the appropriate function default

Rather than hardcoding MTP as the appropriate default when the user
selects "Charger Only", call UsbDeviceManager.setCurrentFunction(null).
Null is intended to restore the default function, which is appropriate
for charging only.

Bug: 21429947
Change-Id: I9ee633bcf2f95f9937861c234ab044873202d41b
1 file changed