Add safety comments to prng_seeder.

These will soon be required by a lint.

Bug: 290018030
Test: m rust
Change-Id: I93db8ce5285714c182a6dcdf9464e5ff4656dc36
diff --git a/prng_seeder/src/cutils_socket.rs b/prng_seeder/src/cutils_socket.rs
index ab2c869..b408be6 100644
--- a/prng_seeder/src/cutils_socket.rs
+++ b/prng_seeder/src/cutils_socket.rs
@@ -19,7 +19,11 @@
 
 pub fn android_get_control_socket(name: &str) -> Result<UnixListener> {
     let name = CString::new(name)?;
+    // SAFETY: name is a valid C string, and android_get_control_socket doesn't retain it after it
+    // returns.
     let fd = unsafe { cutils_socket_bindgen::android_get_control_socket(name.as_ptr()) };
     ensure!(fd >= 0, "android_get_control_socket failed");
+    // SAFETY: android_get_control_socket either returns a valid and open FD or -1, and we checked
+    // that it's not -1.
     Ok(unsafe { UnixListener::from_raw_fd(fd) })
 }