transcoding: use background io priority

bug: 183751395
test: unit tests; atest MediaTranscodeManagerTest
Change-Id: I1d74da88e1db8557f9d6940219e85a2d80d76809
diff --git a/apex/mediatranscoding.rc b/apex/mediatranscoding.rc
index ae9f8ba..87df2d5 100644
--- a/apex/mediatranscoding.rc
+++ b/apex/mediatranscoding.rc
@@ -7,6 +7,7 @@
     group media
     ioprio rt 4
     # Restrict to little cores only with system-background cpuset.
-    writepid /dev/cpuset/system-background/tasks
+    # Lower IO priority to background.
+    writepid /dev/cpuset/system-background/tasks /dev/blkio/background/tasks
     interface aidl media.transcoding
     disabled