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When adb is disabled, the default usb mode would be "none", which
would turn off the driver and prevent UsbDeviceManager from receiving
any new USB connect / disconnect messages. This prevents the user
from ever enabling MTP and sharing data when adb is turned off.

As discussed in bug 21429947, we work around this problem by keeping
the USB driver in MTP mode most of the time, so that we continue to
receive USB connect / disconnect messages.

To avoid leaking confidential user photos, this change honors the
unlock state sent to us by the UsbDeviceManager code. We only expose
user data if explicitly authorized by the user. MTP being enabled
is decoupled from data being exposed on the USB connection.

Bug: 21429947
Change-Id: I495011aa4e3e18c5e5e6fe7b9d0e1a3efe747ee7