Allow RTP module thread checking to know PacketRouter status.

Since https://webrtc-review.googlesource.com/c/src/+/228433 both audio
and video now only call Get/SetRtpState while not registered to the
packet router.

We can thus remove the lock around packet sequencer and just use a
thread checker.

Bug: webrtc:11340
Change-Id: Ie6865cc96c36208700c31a75747ff4dd992ce68d
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/228435
Commit-Queue: Erik Språng <sprang@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#34755}

modules/rtp_rtcp/mocks/mock_rtp_rtcp.h

@@ -115,6 +115,7 @@ class MockRtpRtcpInterface : public RtpRtcpInterface {
               (rtc::ArrayView<const uint16_t> sequence_numbers),
               (const, override));
   MOCK_METHOD(size_t, ExpectedPerPacketOverhead, (), (const, override));
+  MOCK_METHOD(void, OnPacketSendingThreadSwitched, (), (override));
   MOCK_METHOD(RtcpMode, RTCP, (), (const, override));
   MOCK_METHOD(void, SetRTCPStatus, (RtcpMode method), (override));
   MOCK_METHOD(int32_t,