Switch GATT scan timekeeping to use a different clock

System.currentTimeMillis() could cause a scan to have a negative scan
time if the system time changed during a scan. Switch to
SystemClock.elapsedRealtime which is guaranteed to be monotonic.

Test: Start a scan and change system time
Bug: 62057540
Change-Id: Ie4d88df96c0a133955f46c355660756e587d29c5