Compute time differences with Duration

Compute time differences with Duration

Reason why this change is needed:
Currently the durations of fetch events are computed by
registering time instants with System.currentTimeMillis()
and calculating the differences with simple minus operation,
but multiple sources suggest that the best practice is to use
the Java 8 Duration and Instant objects.

What this patch does:
Get time measurements with Instant.now() instead of
System.currentTimeMillis() and calculate the duration of fetch
events (Reachability checks and Negotiation) using
Duration.between().toMillis().

Signed-off-by: Alexa Panfil <alexapizza@google.com>
Change-Id: I573a0a0562070083cf5a5a196d9710f69a7fa587