UsbDeviceManager: Don't unnecessarily touch properties

Before setting the properties persist.sys.usb.config or sys.usb.config,
check the existing values. If the values are the same as what we'd set it
to, don't perform the set.

Any USB property set, even if setting the property to the value it already
has, triggers one of the "on property" triggers in system/core/rootdir/init.usb.rc .
The script then reconfigures the driver unnecessarily, which causes instability
and dropped connections when trying to use ADB.

Avoid this instability by not performing property sets which would have
no effect.

Bug: 21404762
Bug: 18905620
Change-Id: Id3c2543308df994a0114a0661e20ca799c2dc0e8