Use nanoTime() in finalizer watchdog am: e854e4f766 am: 6c6aa8bbaa am: 4f82632877

Original change: https://googleplex-android-review.googlesource.com/c/platform/libcore/+/11560154

Change-Id: I286aee59a1a277c3bc6ce6ecc4267c7bb0b6d6f1