[Tether] Reduce KeepAlive period from 4 to 3 minutes.

The KeepAlive window on the phone-side is 10 minutes; by reducing the
period at which we send KeepAlive heartbeat messages to 3 minutes, we
give the Chrome OS device 3 chances, instead of 2, to ping the phone
before the window expires -- increasing the stability of the Instant
Tethering session.

Bug: b:143982534
Change-Id: I472139bfa88385c34eb2916cb6d1c2777627a006
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1969774
Commit-Queue: Ryan Hansberry <hansberry@chromium.org>
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#726895}

chromeos/components/tether/keep_alive_scheduler.cc

@@ -15,7 +15,7 @@ namespace chromeos {
 namespace tether {
 
 // static
-const uint32_t KeepAliveScheduler::kKeepAliveIntervalMinutes = 4;
+const uint32_t KeepAliveScheduler::kKeepAliveIntervalMinutes = 3;
 
 KeepAliveScheduler::KeepAliveScheduler(
     device_sync::DeviceSyncClient* device_sync_client,