input: add keycombo, a general key combo driver.

Keycombo lets you provide a key up and key down function, and an
optional time delay for key down. The driver will call the key
down function after the specified key combo has been held for the
speicified time delay. After you release the combo, if the key down
has happened, it calls key up.

Change-Id: I6a9a94e96a8f58fadd908fd1dc7944b9102a089f
Signed-off-by: Daniel Rosenberg <drosen@google.com>