Only listen to echo replies

This discards anything that is not an echo reply. Technically, length
cannot be 0, since zero-length packets close the PacketReader (because
FdEventsReader#shoudlProcessZeroLengthPacket has not been overridden
(yet?)).

Test: atest ApfIntegrationTest
Change-Id: Ib7cd9bf819614e66c8b6b7fc15622be5c344a208
diff --git a/tests/cts/net/src/android/net/cts/ApfIntegrationTest.kt b/tests/cts/net/src/android/net/cts/ApfIntegrationTest.kt
index dbb5bb7..e10a06c 100644
--- a/tests/cts/net/src/android/net/cts/ApfIntegrationTest.kt
+++ b/tests/cts/net/src/android/net/cts/ApfIntegrationTest.kt
@@ -169,8 +169,10 @@
         }
 
         override fun handlePacket(recvbuf: ByteArray, length: Int) {
-            assertThat(length).isEqualTo(64)
-            assertThat(recvbuf[0]).isEqualTo(0x81.toByte())
+            // If zero-length or Type is not echo reply: ignore.
+            if (length == 0 || recvbuf[0] != 0x81.toByte()) {
+                return
+            }
             // Only copy the ping data and complete the future.
             val result = recvbuf.sliceArray(8..<length)
             Log.i(TAG, "Received ping reply: ${result.toHexString()}")