Enable fdtrack in system_server.

Spawn a thread that monitors the current fd count, and enables fdtrack
to hunt down leaks when it gets too high.

Bug: http://b/140703823
Test: setprop persist.sys.debug.fdtrack_enable_threshold; stop; start; logcat -c; killall -39 system_server; logcat -d | grep fdtrack
Change-Id: If5831f57d47e6958112abced181f07e18e6a7261

services/java/com/android/server/SystemServer.java

@@ -353,6 +353,12 @@ public final class SystemServer {
      */
     private static native void initZygoteChildHeapProfiling();
 
+
+    /**
+     * Spawn a thread that monitors for fd leaks.
+     */
+    private static native void spawnFdLeakCheckThread();
+
     /**
      * The main entry point from zygote.
      */
@@ -484,6 +490,11 @@ public final class SystemServer {
                 initZygoteChildHeapProfiling();
             }
 
+            // Debug builds - spawn a thread to monitor for fd leaks.
+            if (Build.IS_DEBUGGABLE) {
+                spawnFdLeakCheckThread();
+            }
+
             // Check whether we failed to shut down last time we tried.
             // This call may not return.
             performPendingShutdown();