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.
Change-Id: I0afebd432c5d04c47c93daa1ce616d712aa323d7
diff --git a/tests/net/java/android/net/IpSecManagerTest.java b/tests/net/java/android/net/IpSecManagerTest.java
index cc3366f..d88f8af 100644
--- a/tests/net/java/android/net/IpSecManagerTest.java
+++ b/tests/net/java/android/net/IpSecManagerTest.java
@@ -177,7 +177,7 @@
 
         IpSecManager.UdpEncapsulationSocket encapSocket =
                 mIpSecManager.openUdpEncapsulationSocket(TEST_UDP_ENCAP_PORT);
-        assertNotNull(encapSocket.getSocket());
+        assertNotNull(encapSocket.getFileDescriptor());
         assertEquals(TEST_UDP_ENCAP_PORT, encapSocket.getPort());
 
         encapSocket.close();
@@ -201,7 +201,7 @@
         IpSecManager.UdpEncapsulationSocket encapSocket =
                 mIpSecManager.openUdpEncapsulationSocket();
 
-        assertNotNull(encapSocket.getSocket());
+        assertNotNull(encapSocket.getFileDescriptor());
         assertEquals(TEST_UDP_ENCAP_PORT, encapSocket.getPort());
 
         encapSocket.close();