input: kbd_matrix: add an kbd_matrix_state shell command

Add a "input kbd_matrix_state" shell command. This prints the state of
a keyboard matrix in a much more compact representation than the normal
input event dump, but also keeps track of any key seen during the
execution and reports that on the "off" command. The output can be used
to help setting the actual-key-mask property.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>