Add policy transparency UI to USB connections

Update the preferences for USB connected devices to be of
RestrictedSelectorWithWidgetPreference class to display user restriction
on the UI transparently.

Bug: 330128955
Test: Manual test by device owner provisioning on TestDPC and setting
USB file transfer and config tethering disallowed. Then check Settings.
See recording in http://screencast/cast/NDc5MDMyMjY1ODgwMzcxMnxjNzhhMTQ0Ny02ZA
Flag: EXEMPT bugfix

Change-Id: Ie229a6e68caafec42b80439ac3eb2f45398d23c6