transcoding: restrict to system-background cpuset

Bug: 183751395
Test: builds;unit tests; CTS MediaTranscodeManagerTest.
Change-Id: I2074162b9d289f49d8aec7ad26ec564fa846ac38
diff --git a/apex/mediatranscoding.rc b/apex/mediatranscoding.rc
index fa4acf8..24306a2 100644
--- a/apex/mediatranscoding.rc
+++ b/apex/mediatranscoding.rc
@@ -6,5 +6,6 @@
     user media
     group media
     ioprio rt 4
-    task_profiles ProcessCapacityHigh HighPerformance
+    # Restrict to little cores only with system-background cpuset.
+    writepid /dev/cpuset/system-background/tasks
     disabled