Add owners for KeyChain code

Add the Android Enterprise Security team as OWNERS for KeyChain and
KeyChain-related code.

The KeyChain code currently lives under keystore/, which means every
change requires Keystore owners approval, but it does not make sense for
KeyChain as KeyChain is a Keystore client and is developed
independently.

Test: Gerritt upload.
Bug: 33166666
Change-Id: Idfedda9553add303439179ce10a1e75e437bbe83