Updated the description for APPLICATION_ID and APPLICATION_DATA

As the signature of the getKeyCharacteristics() does not
use Tag Mechanism for app_id and app_data, there is no way
to distinguish between appId / appData  values that are
absent, vs values that are present but of zero length. Due to
this limitation a key with a zero-length app_id / app_data
cannot have its key characteristics retrieved using
getKeyCharacteristics()

Test: VtsAidlKeyMintTarget
Change-Id: I145dcba878171c174d48ad42fadeb49e045b5c55