KeyStore: add API to uid versions

In previous commits, we added the ability to specify which UID we want to
target on certain operations. This commit adds the ability to reach those
binder calls from the KeyStore class.

Also fix a problem where saw() was not reading all the values returned via
the Binder call. This changes the semantics to return a null instead of
failing silently when it's not possible to search.

Change-Id: I32098dc0eb42e09ace89f6b7455766842a72e9f4