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