Modify how USB connections are handled.

* Introduce a new "charger only" mode. In this mode, MTP is disabled,
and no file transfers can occur.
* Make charger only mode the default.
* Modify "persist.sys.usb.config" so it now only holds the adb status.
* Make the USB settings non-persistent. Unplugging the USB connection will
reset the device back to "charger only" mode.
* Fixup wording per UI guidelines.

TODO: Re-implement MDM restrictions for USB / MTP access controls.

Bug: 18905620
Change-Id: I99a50d9132a81e98187f431166fd9fef4d437e4f