Fix janky navbar ripples -- incorrect timerslack values

If a thread is created while the parent thread is "Background",
then the default timerslack value gets set to the current
timerslack value of the parent (40ms). The default value is
used when transitioning to "Foreground" -- so the effect is that
the timerslack value becomes 40ms regardless of foreground/background.

This does occur intermittently for systemui when creating its
render thread (pretty often on hammerhead and has been seen on
shamu). If this occurs, then some systemui animations like navbar
ripples can wait for up to 40ms to draw a frame when they intended
to wait 3ms -- jank.

This fix is to explicitly set the foreground timerslack to 50us.

A consequence of setting timerslack behind the process' back is
that any custom values for timerslack get lost whenever the thread
has transition between fg/bg.

See Bug: 19398120

Change-Id: Idc259717f62fa2255f8bafbbf88b68c0043f29cf
diff --git a/libcutils/sched_policy.c b/libcutils/sched_policy.c
index a7ff85e..1ffe872 100644
--- a/libcutils/sched_policy.c
+++ b/libcutils/sched_policy.c
@@ -50,6 +50,7 @@
 // timer slack value in nS enforced when the thread moves to background
 #define TIMER_SLACK_BG 40000000
+#define TIMER_SLACK_FG 50000
 static pthread_once_t the_once = PTHREAD_ONCE_INIT;
@@ -356,7 +357,8 @@
-    prctl(PR_SET_TIMERSLACK_PID, policy == SP_BACKGROUND ? TIMER_SLACK_BG : 0, tid);
+          policy == SP_BACKGROUND ? TIMER_SLACK_BG : TIMER_SLACK_FG, tid);
     return 0;