Input: defer handler's start() until device is opened

When registering an input handle, handler->start() is currently called
immediately. However, the input device might not be fully opened or
ready to process events at this stage, meaning any state synchronization
events (like setting LED states) injected by the handler's start method
might be dropped.

Move the handler->start() invocation to input_open_device(). If it is
the first handle opening the device, start() is called after the driver's
open() method has successfully completed and the device is fully prepared.

To facilitate this, factor out the device startup logic (calling driver's
open and starting polling) into input_start_device().

For passive observer handlers, their start() method is also deferred
until the handle is opened. Since opening a passive observer handle does
not start the underlying hardware device, their start() method is called
immediately upon opening, regardless of whether the device is active.

Fixes: c7e8dc6ee6d5 ("Input: add start() method to input handlers")
Link: https://patch.msgid.link/20260803005210.1251102-4-dmitry.torokhov@gmail.com
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

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