Fix rtp_analyzer tool

- Make rtp_analyzer work with a single SSRC
- Simplify rtp_analyzer.sh (it allows to run the python script
  from any directory)
- Update README.md (simplified, added missing dependency)

Bug: webrtc:10829
Change-Id: Idb82e7228918a973778762a39b732ce3b26b6bbc
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/146711
Reviewed-by: Artem Titov <titovartem@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Alessio Bazzica <alessiob@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#28668}

rtc_tools/py_event_log_analyzer/rtp_analyzer.sh

@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/bin/bash
 #  Copyright (c) 2016 The WebRTC project authors. All Rights Reserved.
 #
 #  Use of this source code is governed by a BSD-style license
@@ -6,12 +6,5 @@
 #  tree. An additional intellectual property rights grant can be found
 #  in the file PATENTS.  All contributing project authors may
 #  be found in the AUTHORS file in the root of the source tree.
-set -e
-WORKING_DIR=$(pwd)
-cd $(dirname $0)
-PYTHONPATH="../../third_party/protobuf/python/"
-if [ -z ${PYTHON_EXECUTABLE+x} ]
-then
-    PYTHON_EXECUTABLE=python3
-fi
-exec $PYTHON_EXECUTABLE "rtp_analyzer.py" $@ --working_dir $WORKING_DIR
+BASE_DIR=`dirname $0`
+python "${BASE_DIR}/rtp_analyzer.py" $@ --working_dir $BASE_DIR