Transcoder: Request background priority mode from muxer and codecs.

Request MediaCodec and MediaMuxer to run their internal threads
on background priority.

Bug: 183751395
Test: build_and_run_all_unit_tests.sh and manually inspecting format
Change-Id: I7982874d7155c95f38a9bfed9ed9c3a87acf445f
diff --git a/media/libmediatranscoding/transcoder/tests/MediaSampleWriterTests.cpp b/media/libmediatranscoding/transcoder/tests/MediaSampleWriterTests.cpp
index 8b3905c..0a8a06e 100644
--- a/media/libmediatranscoding/transcoder/tests/MediaSampleWriterTests.cpp
+++ b/media/libmediatranscoding/transcoder/tests/MediaSampleWriterTests.cpp
@@ -117,8 +117,9 @@
 }
 
 bool operator==(const TestMuxer::Event& lhs, const TestMuxer::Event& rhs) {
-    return lhs.type == rhs.type && lhs.format == rhs.format && lhs.trackIndex == rhs.trackIndex &&
-           lhs.data == rhs.data && lhs.info == rhs.info;
+    // Don't test format pointer equality since the writer can make a copy.
+    return lhs.type == rhs.type /*&& lhs.format == rhs.format*/ &&
+           lhs.trackIndex == rhs.trackIndex && lhs.data == rhs.data && lhs.info == rhs.info;
 }
 
 /** Represents a media source file. */