UsbDeviceManager: Modify default function handling

When adb is enabled, there's no need to also enable MTP mode.
ADB mode enabled is sufficient to get connect messages from
the kernel driver.

This change modifies getDefaultFunctions() to return adb mode
if adb is enabled, but mtp when it's not enabled. Also stop
using "none" mode as the default. "none" should only be used
when toggling the driver.

Bug: 21429947
Change-Id: I767491417c6c69a120fcf4f3d6ec0c73311a1a60