installd: Set priority to background for dexopting

This fixes jank caused by dex2oat while installing an application in the
background.

Bug: 17497551
Change-Id: I5a69b00c0fd76ae22a0d1adb242bef6c18a75743
diff --git a/cmds/installd/commands.c b/cmds/installd/commands.c
index fc3972e..1954d8d 100644
--- a/cmds/installd/commands.c
+++ b/cmds/installd/commands.c
@@ -20,6 +20,7 @@
 #include <cutils/sched_policy.h>
 #include <diskusage/dirsize.h>
 #include <selinux/android.h>
+#include <system/thread_defs.h>
 
 /* Directory records that are used in execution of commands. */
 dir_rec_t android_data_dir;
@@ -985,6 +986,10 @@
             ALOGE("set_sched_policy failed: %s\n", strerror(errno));
             exit(70);
         }
+        if (setpriority(PRIO_PROCESS, 0, ANDROID_PRIORITY_BACKGROUND) < 0) {
+            ALOGE("setpriority failed: %s\n", strerror(errno));
+            exit(71);
+        }
         if (flock(out_fd, LOCK_EX | LOCK_NB) != 0) {
             ALOGE("flock(%s) failed: %s\n", out_path, strerror(errno));
             exit(67);