transcoding: change service thread priority to background

bug: 183751395
test: unit tests; CTS MediaTranscodeManagerTest
Change-Id: I3b8922838ec633af2e1e7b67ad9eac832ba88a04
diff --git a/media/libmediatranscoding/TranscoderWrapper.cpp b/media/libmediatranscoding/TranscoderWrapper.cpp
index b19e711..5e4c671 100644
--- a/media/libmediatranscoding/TranscoderWrapper.cpp
+++ b/media/libmediatranscoding/TranscoderWrapper.cpp
@@ -23,6 +23,7 @@
 #include <media/NdkCommon.h>
 #include <media/TranscoderWrapper.h>
 #include <media/TranscodingRequest.h>
+#include <utils/AndroidThreads.h>
 #include <utils/Log.h>
 
 #include <thread>
@@ -599,6 +600,7 @@
 }
 
 void TranscoderWrapper::threadLoop() {
+    androidSetThreadPriority(0 /*tid (0 = current) */, ANDROID_PRIORITY_BACKGROUND);
     std::unique_lock<std::mutex> lock{mLock};
     // TranscoderWrapper currently lives in the transcoding service, as long as
     // MediaTranscodingService itself.