init: prevent persist.sys.usb.config initalized as none,adb

Prevent appending ",adb" to persist.sys.usb.config if "none" is
explicitly defined in default prop.

Bug: 150130503
Test: persist.sys.usb.config initalized correctly
Change-Id: I3b5de6fd102e252019e843f39f0875f5aaea7486
1 file changed