[CRD iOS] Attach the bottom of the connection view icon to the center of the view

Rather than using the fixed kTopPadding. This ensures the UI is mostly
centered in screen size and orientation.

Change-Id: Ia78f020a84566873ceb08f837917b5bf722ab4af
Reviewed-on: https://chromium-review.googlesource.com/571682Reviewed-by: Scott Nichols <nicholss@chromium.org>
Commit-Queue: Yuwei Huang <yuweih@chromium.org>
Cr-Commit-Position: refs/heads/master@{#486956}

remoting/ios/app/client_connection_view_controller.mm

@@ -36,7 +36,6 @@ static const CGFloat kPinEntryViewHeight = 90.f;
 static const CGFloat kReconnectViewWidth = 120.f;
 static const CGFloat kReconnectViewHeight = 90.f;
 
-static const CGFloat kTopPadding = 240.f;
 static const CGFloat kPadding = 20.f;
 static const CGFloat kMargin = 20.f;
 
@@ -197,7 +196,6 @@ static const CGFloat kKeyboardAnimationTime = 0.3;
   NSDictionary* layoutMetrics = @{
     @"padding" : @(kPadding),
     @"margin" : @(kMargin),
-    @"topPadding" : @(kTopPadding),
     @"iconDiameter" : @(kIconRadius * 2),
     @"pinEntryViewWidth" : @(kPinEntryViewWidth),
     @"pinEntryViewHeight" : @(kPinEntryViewHeight),
@@ -237,9 +235,8 @@ static const CGFloat kKeyboardAnimationTime = 0.3;
                                                       views:views]];
 
   // Anchors:
-  _activityIndicatorTopConstraintFull =
-      [_activityIndicator.topAnchor constraintEqualToAnchor:self.view.topAnchor
-                                                   constant:kTopPadding];
+  _activityIndicatorTopConstraintFull = [_activityIndicator.bottomAnchor
+      constraintEqualToAnchor:self.view.centerYAnchor];
   _activityIndicatorTopConstraintFull.active = YES;
 
   [_iconView.centerYAnchor