Transcoder: Preserve source track duration.

Preserve the source track's duration by sending an
EOS sample with the appropriate PTS to the muxer.
Modified MediaSampleWriterTests to verify the extra
EOS sample.

Bug: 159732935
Test: MediaTranscoder and MediaSampleWriter unit tests.
Change-Id: I6792eff8932db126e8734e5d3d6fe8aee2ffb4a1
diff --git a/media/libmediatranscoding/transcoder/tests/MediaTranscoderTests.cpp b/media/libmediatranscoding/transcoder/tests/MediaTranscoderTests.cpp
index 67f1ca1..7cb435b 100644
--- a/media/libmediatranscoding/transcoder/tests/MediaTranscoderTests.cpp
+++ b/media/libmediatranscoding/transcoder/tests/MediaTranscoderTests.cpp
@@ -203,6 +203,7 @@
 
         std::shared_ptr<MediaSampleReader> sampleReader =
                 MediaSampleReaderNDK::createFromFd(dstFd, 0, fileSize);
+        ASSERT_NE(sampleReader, nullptr);
 
         std::shared_ptr<AMediaFormat> videoFormat;
         const size_t trackCount = sampleReader->getTrackCount();
@@ -211,6 +212,7 @@
             if (trackFormat != nullptr) {
                 const char* mime = nullptr;
                 AMediaFormat_getString(trackFormat, AMEDIAFORMAT_KEY_MIME, &mime);
+
                 if (strncmp(mime, "video/", 6) == 0) {
                     LOG(INFO) << "Track # " << trackIndex << ": "
                               << AMediaFormat_toString(trackFormat);