Add, standardise or temporarily opt out of safety comments for keystore2.

These will soon be required by a lint.

Some functions were incorrectly marked as safe which were not actually
safe, so I've fixed those too.

Bug: 290018030
Test: m rust
Change-Id: I38df6a8162d430617f123ab1aace38b741458fce
diff --git a/keystore2/selinux/src/lib.rs b/keystore2/selinux/src/lib.rs
index e5c3091..32fdb59 100644
--- a/keystore2/selinux/src/lib.rs
+++ b/keystore2/selinux/src/lib.rs
@@ -20,6 +20,9 @@
 //!  * selabel_lookup for the keystore2_key backend.
 //! And it provides an owning wrapper around context strings `Context`.
 
+// TODO(b/290018030): Remove this and add proper safety comments.
+#![allow(clippy::undocumented_unsafe_blocks)]
+
 use anyhow::Context as AnyhowContext;
 use anyhow::{anyhow, Result};
 use lazy_static::lazy_static;
@@ -160,8 +163,9 @@
     handle: *mut selinux::selabel_handle,
 }
 
-// KeystoreKeyBackend is Sync because selabel_lookup is thread safe.
+// SAFETY: KeystoreKeyBackend is Sync because selabel_lookup is thread safe.
 unsafe impl Sync for KeystoreKeyBackend {}
+// SAFETY: KeystoreKeyBackend is Send because selabel_lookup is thread safe.
 unsafe impl Send for KeystoreKeyBackend {}
 
 impl KeystoreKeyBackend {