Fix Shell page transition color blinking on Android

[Copilot]

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Description of Change

Shell page transitions flash white on dark-themed apps because BottomNavigationView is created with hardcoded Color.WHITE in PlatformInterop.java, and navigation containers (FrameLayout, CoordinatorLayout) have no initial background. The theme-aware color only gets applied later via SetAppearance(), causing visible flash during the transition window.

Fix: Set ShellRenderer.DefaultBottomNavigationViewBackgroundColor immediately after view creation:

• ShellItemRenderer - Apply to _bottomView and _navigationArea
• ShellSectionRenderer - Apply to root CoordinatorLayout
• ShellContentFragment - Apply to inflated CoordinatorLayout

var themeAwareBackgroundColor = ShellRenderer.DefaultBottomNavigationViewBackgroundColor.ToPlatform();
_bottomView.SetBackgroundColor(themeAwareBackgroundColor);
_navigationArea.SetBackgroundColor(themeAwareBackgroundColor);

DefaultBottomNavigationViewBackgroundColor returns white for light theme, #1B3147 for dark theme.

Issues Fixed

Fixes #16621

Original prompt

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This section details on the original issue you should resolve

<issue_title>Shell page transition animation causes color blinking</issue_title>
<issue_description>### Description

Shell page transition causes a severe color blinking, especially for a dark themed application. It looks like the application uses default, light system theme when entering the page and then applying currently selected theme from application. See the gif below. The white blink is visible on the entire screen, furthermore the tabbar color transition from white to black is visible as well.

Occurs for both navigation invocation:

await Shell.Current.GoToAsync($"//{Routes.WelcomePage}", false);
await Shell.Current.GoToAsync($"//{Routes.WelcomePage}", true);

Tested on .NET: 8.0.100-preview.7.23376.3
MAUI: 8.0.0-preview.6.8686 and 8.0.0-preview.7.8842

In my opinion, expected behaviour is to either apply theme changing once it is triggered and do not invoke such color transition animations during further page transitions or add Shell navigation parameter to enable/disable the animation. Currently existed Shell.Current.GoToAsync "animate" parameter seems not to be respected at all.

Steps to Reproduce

Download the code from the public repo
Deploy onto an Android simulator and observe the problem.

Link to public reproduction project repository

https://github.com/Totalq09/MAUITabBarProblem

Version with bug

8.0.0-preview.6.8686

Last version that worked well

Unknown/Other

Affected platforms

Android

Affected platform versions

Tested on Android 13 and Android 9 with the same effect

Did you find any workaround?

No, even custom ShellRenderer did not help

Relevant log output

No response

</issue_description>

Comments on the Issue (you are @copilot in this section)

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• Fixes Shell page transition animation causes color blinking #16621

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