Tweaks to Onboarding following design review.

Use constants for consistent top padding and icon sizes.
Match layout of AnalyticsPrompt on iPad to other views.
Update icon in use case selection screen to allow tinting.

RiotSwiftUI/Modules/Onboarding/DisplayName/View/OnboardingDisplayNameScreen.swift

@@ -49,9 +49,9 @@ struct OnboardingDisplayNameScreen: View {
                 
                 buttons
             }
-            .padding(.horizontal)
-            .padding(.top, 8)
             .frame(maxWidth: OnboardingMetrics.maxContentWidth)
+            .padding(.horizontal)
+            .padding(.top, OnboardingMetrics.topPaddingToNavigationBar)
         }
         .frame(maxWidth: .infinity, maxHeight: .infinity)
         .accentColor(theme.colors.accent)
@@ -69,7 +69,7 @@ struct OnboardingDisplayNameScreen: View {
                 .resizable()
                 .renderingMode(.template)
                 .foregroundColor(theme.colors.accent)
-                .frame(width: 90, height: 90)
+                .frame(width: OnboardingMetrics.iconSize, height: OnboardingMetrics.iconSize)
                 .background(Circle().foregroundColor(.white).padding(2))
                 .padding(.bottom, 8)
                 .accessibilityHidden(true)