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/km_compat/lib.rs b/keystore2/src/km_compat/lib.rs
index 2632ec4..e61a13a 100644
--- a/keystore2/src/km_compat/lib.rs
+++ b/keystore2/src/km_compat/lib.rs
@@ -21,6 +21,7 @@
 
 #[allow(missing_docs)] // TODO remove this
 pub fn add_keymint_device_service() -> i32 {
+    // SAFETY: This is always safe to call.
     unsafe { addKeyMintDeviceService() }
 }