hid: Add driver for FiiO USB DAC

The FiiO E18 USB DAC generates multiple key presses immediately
after being plugged in, but doesn't generate all of the corresponding
key up events.  Handle this peculiarity by only passing events that
we have seen both a keypress and release for.

CYNGNOS-697
Change-Id: I654f55ac7376a82551e1c7eeadb8f9f3a6d474d3
(cherry picked from commit 0a7d1cb54af43dd19d38843fbb5c846cbd133498)