Input: introduce notion of passive observers for input handlers

Sometimes it is useful to observe (and maybe modify) data coming from
an input device, but only do that if there are other users of such input
device. An example is touchpad switching functionality on Lenovo IdeaPad
Z570 where it is desirable to suppress events coming from the touchpad
if user toggles touchpad on/off button (on this laptop the firmware does
not stop the device).

Introduce notion of passive observers for input handlers to solve this
issue. An input handler marked as passive observer behaves exactly like
any other input handler or filter, but with one exception: it does not
open/start underlying input device when attaching to it.

Link: https://lore.kernel.org/r/ZxlEROX7bMo5cbZP@google.com
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

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