usb: serial: Add SoftAP + DUN flags to serial device interface

Added an 'SoftAP+DUN enabled' attribute to the Android serial device.
This flag determines whether we are in SoftAP+DUN configuration.
Added an 'SoftAP+DUN active' attribute to the Android serial device.
This flag determines whether a USB connection alive during
SoftAP+DUN mode.

Change-Id: I979417974c8564766504547b1f1842aeb1128567
Signed-off-by: Danny Segal <dsegal@codeaurora.org>
Signed-off-by: Lena Salman <esalman@codeaurora.org>