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/NdkCommon.cpp b/media/libmediatranscoding/transcoder/NdkCommon.cpp
index 2d85df7..e133cd6 100644
--- a/media/libmediatranscoding/transcoder/NdkCommon.cpp
+++ b/media/libmediatranscoding/transcoder/NdkCommon.cpp
@@ -42,6 +42,7 @@
 
 /* TODO(lnilsson): Finalize value or adopt AMediaFormat key once available. */
 const char* TBD_AMEDIACODEC_PARAMETER_KEY_COLOR_TRANSFER_REQUEST = "color-transfer-request";
+const char* TBD_AMEDIACODEC_PARAMETER_KEY_BACKGROUND_MODE = "android._background-mode";
 
 namespace AMediaFormatUtils {