fdtrack: add stats logging.

Bug: http://b/183008833
Bug: http://b/183018597
Test: adb shell 'setprop persist.sys.debug.fdtrack_interval 10; stop; start' && statsd_testdrive 352
Change-Id: Ibc2293f8f1c7f0f29a166ac8dc93afc8dfd8b7de
Merged-In: Ibc2293f8f1c7f0f29a166ac8dc93afc8dfd8b7de
(cherry picked from commit 53731ea9ab0c131fb51e891946a17895571973d1)
1 file changed