NetEq: Change member variables for current RTP types to rtc::Optionals

With this change, the value 0xFF is no longer used to flag that the RTP
type is unknown. Instead, an empty value for the rtc::Optional is used.

Review-Url: https://codereview.webrtc.org/2290153002
Cr-Commit-Position: refs/heads/master@{#13989}

webrtc/modules/audio_coding/neteq/neteq_impl_unittest.cc

@@ -287,8 +287,9 @@ TEST_F(NetEqImplTest, InsertPacket) {
       .Times(1);
   EXPECT_CALL(*mock_packet_buffer_, InsertPacketList(_, _, _, _))
       .Times(2)
-      .WillRepeatedly(DoAll(SetArgPointee<2>(kPayloadType),
-                            WithArg<0>(Invoke(DeletePacketsAndReturnOk))));
+      .WillRepeatedly(
+          DoAll(SetArgPointee<2>(rtc::Optional<uint8_t>(kPayloadType)),
+                WithArg<0>(Invoke(DeletePacketsAndReturnOk))));
   // SetArgPointee<2>(kPayloadType) means that the third argument (zero-based
   // index) is a pointer, and the variable pointed to is set to kPayloadType.
   // Also invoke the function DeletePacketsAndReturnOk to properly delete all