Fix deviced hang problem when changing usb debug menu quickly

Fix deviced hang problem when changing usb debug menu quickly

If you change the usb debug menu quickly,
a new vconf request comes in the middle of the vconf change and the state is twisted.
As a result, deviced process the vconf handler indefinitely.

If you change the usb debug menu slowly, deviced process vconfs in order, so there is no problem.

Solution: when changing the usb mode through the usb debug menu, do not change the usb debug vconf.

Change-Id: Ie274844fc306c2bf7c3b873a66c338d635d9b760
Signed-off-by: Youngjae Cho <y0.cho@samsung.com>