Prevent mixing mutable and immutable lists

As per errorprone feedback.

Test: TH
Change-Id: I4da5673551e4c8c4b3fea0aa65d34649126aaff8
diff --git a/service-t/src/com/android/server/ethernet/EthernetTracker.java b/service-t/src/com/android/server/ethernet/EthernetTracker.java
index def701b..8f000a3 100644
--- a/service-t/src/com/android/server/ethernet/EthernetTracker.java
+++ b/service-t/src/com/android/server/ethernet/EthernetTracker.java
@@ -1037,7 +1037,7 @@
                 }
                 capabilities.add(capability);
             }
-            return capabilities;
+            return Collections.unmodifiableList(capabilities);
         }
 
         private static int parseTransportType(String transportString) {