[CrOS Tether] Cover all edge cases for BLE connections.

We've seen some recent crashes (e.g., crbug.com/756188) which result
from the BluetoothLowEnergyWeaveClientConnection class. I have not been
able to track down exactly how this crash is caused, but with this CL, I
cover all possible edge cases which could cause a crash and added lots
of extra logging which will help us catch this issue.

For now, this CL should prevent a crash while we continue to debug.

Bug: 756188, 672263
Change-Id: Ibe8f406bd830fb7ed6b7fb21d61be05068e81cea
Reviewed-on: https://chromium-review.googlesource.com/634207
Commit-Queue: Kyle Horimoto <khorimoto@chromium.org>
Reviewed-by: Tim Song <tengs@chromium.org>
Cr-Commit-Position: refs/heads/master@{#497318}

chromeos/components/tether/disconnect_tethering_operation.cc

@@ -94,13 +94,14 @@ void DisconnectTetheringOperation::OnMessageSent(int sequence_number) {
     return;
 
   has_sent_message_ = true;
-  UnregisterDevice(remote_device_);
 
   DCHECK(!disconnect_start_time_.is_null());
   UMA_HISTOGRAM_TIMES(
       "InstantTethering.Performance.DisconnectTetheringRequestDuration",
       clock_->Now() - disconnect_start_time_);
   disconnect_start_time_ = base::Time();
+
+  UnregisterDevice(remote_device_);
 }
 
 void DisconnectTetheringOperation::SetClockForTest(