[usb-moded] Support broken android kernels

Some android kernels don't have working cable detection and charging, unless
a gadget is active. This broken by design behaviouris usually easy to fix
(and recommended due to possible charging issues and extra power drain).
But to make initial porting easier we keep the gadget in charging mode,
even when disconnected.

Signed-off-by: Philippe De Swert <philippe.deswert@jollamobile.com>
[usb-moded] Update and beautify usage information

Add option for broken android usb, and make the text line up nicely
for improved readability

Signed-off-by: Philippe De Swert <philippe.deswert@jollamobile.com>
[usb-moded] Document broken android kernel behaviour

Documented what happens when the -a switch is used to make
sure usb_moded works when the kernel cable detection/charging
is "broken" by non-active android gadgets.

Signed-off-by: Philippe De Swert <philippe.deswert@jollamobile.com>