Allow system_server to update timerslack_ns for hal_audio_default

Based on the following audit message:

type=1400 audit(1521738979.005:385): avc: denied { write } for pid=1269
comm="Binder:1269_B" name="timerslack_ns" dev="proc" ino=254190
scontext=u:r:system_server:s0 tcontext=u:r:hal_audio_default:s0
tclass=file permissive=1

Bug: 74110604
Test: adb shell dmesg | grep hal_audio_default
Change-Id: I4c2e787588eb9d223d5e50e1bc8f67876de97c2e

private/system_server.te

@@ -114,6 +114,7 @@ allow system_server bootanim:process { getsched setsched };
 allow system_server appdomain:file w_file_perms;
 allow system_server audioserver:file w_file_perms;
 allow system_server cameraserver:file w_file_perms;
+allow system_server hal_audio_server:file w_file_perms;
 
 # Read /proc/pid data for all domains. This is used by ProcessCpuTracker
 # within system_server to keep track of memory and CPU usage for