Clarify UDP encapsulation socket API

This change updates the getSocket() methods for IPsec to improve clarity
of the return types, both for public APIs, and internal-only methods.

Bug: 72473753
Test: APIs updated, CTS + unit tests ran.
Merged-In: I0afebd432c5d04c47c93daa1ce616d712aa323d7
Change-Id: I0afebd432c5d04c47c93daa1ce616d712aa323d7
(cherry picked from commit 4c987ebade580d4abc8a3d549e0df90baab33140)

services/core/java/com/android/server/IpSecService.java

@@ -931,7 +931,7 @@ public class IpSecService extends IIpSecService.Stub {
             return mPort;
         }
 
-        public FileDescriptor getSocket() {
+        public FileDescriptor getFileDescriptor() {
             return mSocket;
         }