Make NetworkMonitor less aggressive on broken networks.

1. Retry validation 3 times instead of 10.

Reducing the number of retries speeds up dead network detection
and saves battery on networks that are don't have Internet access
(or block our connectivity checks).

Retrying 10 times is overkill. Each attempt sends 5-10 DNS
requests, waits 10 seconds to transfer only 3 or 4 packets, and
can last up to tens of seconds. Also, in MR1 we now retry every
10 minutes.

2. Make periodic revalidation only try once.

When validating periodically to see if a non-working connection
is working again, only make one attempt every 10 minutes, not
three.

3. Rename "retries" to "attempts" to make things a bit clearer.

Bug: 18922569
Bug: 18869302
Change-Id: I5f70b420c4f1bac0c33bb9ab6afb322902cdac4b