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/src/keystore2_main.rs b/keystore2/src/keystore2_main.rs
index 31c1e29..059d59d 100644
--- a/keystore2/src/keystore2_main.rs
+++ b/keystore2/src/keystore2_main.rs
@@ -68,6 +68,8 @@
     fn sqlite_log_handler(err: c_int, message: &str) {
         log::error!("[SQLITE3] {}: {}", err, message);
     }
+    // SAFETY: There are no other threads yet, `sqlite_log_handler` is threadsafe, and it doesn't
+    // invoke any SQLite calls.
     unsafe { sqlite_trace::config_log(Some(sqlite_log_handler)) }
         .expect("Error setting sqlite log callback.");