Improve KeyImpl & KeyAesImpl code coverage

Improve KeyImpl & KeyAesImpl code coverage

Also unify key API.

KeyAesImpl() will now return an empty object instead of throwing. This will
unify the error code returned for symmetric and asymmetric keys from
ckmc_get_key(). It will also fix asynchronous C++ API. Observer will receive
an empty key instead of not being called at all.

Unify the type returned from empty keys. C++ API is a platform one and
getType() function is not used in tizen.org according to CodeGrok.

Change-Id: I7de8f32dfe59b1c5af441dfb9a0b8bee5c0d0bcf