Transcoder: Run all transcoder threads on background priority.

Bug: 183751395
Test: build_and_run_all_unit_tests.sh
Change-Id: I4e30e3fda4b9ed8773991d7d4bff5229df57b52d
diff --git a/media/libmediatranscoding/transcoder/MediaTrackTranscoder.cpp b/media/libmediatranscoding/transcoder/MediaTrackTranscoder.cpp
index 15f7427..f01a948 100644
--- a/media/libmediatranscoding/transcoder/MediaTrackTranscoder.cpp
+++ b/media/libmediatranscoding/transcoder/MediaTrackTranscoder.cpp
@@ -20,6 +20,7 @@
 #include <android-base/logging.h>
 #include <media/MediaTrackTranscoder.h>
 #include <media/MediaTrackTranscoderCallback.h>
+#include <utils/AndroidThreads.h>
 
 namespace android {
 
@@ -72,6 +73,7 @@
     mState = STARTED;
 
     std::thread([this] {
+        androidSetThreadPriority(0 /* tid (0 = current) */, ANDROID_PRIORITY_BACKGROUND);
         bool stopped = false;
         media_status_t status = runTranscodeLoop(&stopped);