[MultiDevice] SecureChannel: Re-enable BLE low connection latency param.

This restores previous Smart Lock behavior (regressed in M71),
improving latency.

Before (average of 10 local runs):
* StartScanToReceiveUnlockableRemoteStatus:		9007.0 ms
* StartScanToAuthentication:				8827.2 ms
* AuthenticationToReceiveUnlockableRemoteStatus:	 179.3 ms

After  (average of 10 local runs):
* StartScanToReceiveUnlockableRemoteStatus:		3685.6 ms
* StartScanToAuthentication:				3576.6 ms
* AuthenticationToReceiveUnlockableRemoteStatus:	 108.6 ms

((9007.0 - 3685.6) / 9007.0) = 59% reduction in overall latency.

Bug: 905724
Change-Id: Ic9dd8e3d11bd3f4140d68bc920a96f2822fc3519
Reviewed-on: https://chromium-review.googlesource.com/c/1448987Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Commit-Queue: Ryan Hansberry <hansberry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#628422}

chromeos/services/secure_channel/ble_connection_manager_impl.cc

@@ -346,7 +346,7 @@ void BleConnectionManagerImpl::OnReceivedAdvertisement(
       weave::BluetoothLowEnergyWeaveClientConnection::Factory::NewInstance(
           remote_device, bluetooth_adapter_,
           device::BluetoothUUID(kGattServerUuid), bluetooth_device,
-          false /* should_set_low_connection_latency */);
+          true /* should_set_low_connection_latency */);
 
   SetAuthenticatingChannel(
       remote_device.GetDeviceId(),

chromeos/services/secure_channel/ble_connection_manager_impl_unittest.cc

@@ -222,7 +222,7 @@ class FakeWeaveClientConnectionFactory
       bool should_set_low_connection_latency) override {
     EXPECT_EQ(expected_mock_adapter_, adapter);
     EXPECT_EQ(device::BluetoothUUID(kGattServerUuid), remote_service_uuid);
-    EXPECT_FALSE(should_set_low_connection_latency);
+    EXPECT_TRUE(should_set_low_connection_latency);
 
     auto instance = std::make_unique<FakeConnection>(remote_device);
     last_created_instance_ = instance.get();