Migrate LeRandCallback to be base::OnceCallback

base::Bind and base::Callback are deprecated. Migrate the usages to
proper replacements.

The callback of LeRand is expected to be called for receiving the
resulting random number. It's called only once so it should be a
base::OnceCallback.

Bug: 272116782
Test: m .
Change-Id: I8dc78f54aae455f089bc29ee814c3e0c0635ba9e