Store elapsedRealtimeNs as int64 instead of long

long can be 32 bits, which will overflow in ~2 seconds if signed.

Test: builds
Change-Id: I78e69e2c2ceb21a5748d9d5d860b45e5a3fae865
Merged-In: I78e69e2c2ceb21a5748d9d5d860b45e5a3fae865