Fix/improve tracking of latency on page loads.

[dbrant]

We're currently using System.currentTimeMillis() for tracking the latency of page loads, which is actually not the appropriate thing to use in this case.
The currentTimeMillis() function gives us the "current time" of the device, which can actually change at any moment:

• The device's clock might have drifted a bit, and suddenly receives an updated date/time from GPS or the network.
• The user moves from one time zone into another.
• The user changes the date/time on the device manually.

A much more appropriate function for measuring an "elapsed" time is the nanoTime() function, which gives the total nanoseconds of uptime for the current VM.

(Note that we should not be changing all instances of currentTimeMillis() into nanoTime(), since there are cases where currentTimeMillis() is more appropriate.)