Add get_sched_policy_name() and use in ps and top

This will make it easier to add additional policies (cgroups) if needed.
Also added comments to the sched_policy APIs.

Change-Id: I33ce1cc4deae10983241f7391294b7a512d2c47c
diff --git a/libcutils/sched_policy.c b/libcutils/sched_policy.c
index f9c111e..35d362a 100644
--- a/libcutils/sched_policy.c
+++ b/libcutils/sched_policy.c
@@ -273,5 +273,17 @@
     return 0;
 }
 
+const char *get_sched_policy_name(SchedPolicy policy)
+{
+    static const char * const strings[SP_CNT] = {
+       [SP_BACKGROUND] = "bg",
+       [SP_FOREGROUND] = "fg",
+    };
+    if ((policy < SP_CNT) && (strings[policy] != NULL))
+        return strings[policy];
+    else
+        return "error";
+}
+
 #endif /* HAVE_PTHREADS */
 #endif /* HAVE_SCHED_H */