Refactor key code mapping.

Added handling for EV_MSC / MSC_SCAN which typically reports
the HID usage associated with a key.  This will enable key maps
to map keys with HID usages that Linux does not natively recognize.

Removed keyCode and flags fields from EventHub RawEvent since
they don't necessarily make sense in isolation now that we
pay attention to HID usage codes too.

Removed the fallback code for mapping keys and axes.  In practice,
an input device should be self-sufficient.  We should not ever
need to look at the built-in keyboard's key map.  In fact, there
usually isn't a built-in keyboard anyhow.  This code was originally
working around a problem where we weren't loading the key map
for touch screens with virtual keys, which has long since been fixed.

Change-Id: I0a319bdec44be9514f795526347397e94d53a127