[HybridWebView] fix trimmer warnings on Android

[jonathanpeppers]

Context: #23769
Context: dotnet/android#9300

026e046 introduced HybridWebView, which unfortunately introduces trimmer warnings in the dotnet new maui project template:

> dotnet new maui
> dotnet build -f net9.0-android -c Release -p:TrimMode=Full
...
hellomaui succeeded with 1 warning(s) (7.9s)
/_/src/Core/src/Handlers/HybridWebView/HybridWebViewHandler.Android.cs(53,5):
Trim analysis warning IL2026: Microsoft.Maui.Handlers.HybridWebViewHandler.HybridWebViewJavaScriptInterface.SendMessage(String):
Using member 'Java.Interop.ExportAttribute.ExportAttribute(String)' which has 'RequiresUnreferencedCodeAttribute' can break functionality when trimming application code.
[ExportAttribute] uses dynamic features.

This is due to usage of Java.Interop.ExportAttribute:

private sealed class HybridWebViewJavaScriptInterface : Java.Lang.Object
{
    //...
    [JavascriptInterface]
    [Export("sendMessage")]
    public void SendMessage(string message)

Java.Interop.ExportAttribute makes heavy usage of unbounded System.Reflection, System.Reflection.Emit, etc. for it to be able to work. It brings in an additional assembly Mono.Android.Export.dll as well.

It is inherently trimming unsafe, but how did it get through these tests?

maui/src/TestUtils/src/Microsoft.Maui.IntegrationTests/AndroidTemplateTests.cs

Lines 50 to 61 in 08ff124

	[Test]
	[TestCase("maui", DotNetPrevious, "Debug", null)]
	[TestCase("maui", DotNetPrevious, "Release", null)]
	[TestCase("maui", DotNetCurrent, "Debug", null)]
	[TestCase("maui", DotNetCurrent, "Release", null)]
	[TestCase("maui", DotNetCurrent, "Release", "full")]
	[TestCase("maui-blazor", DotNetPrevious, "Debug", null)]
	[TestCase("maui-blazor", DotNetPrevious, "Release", null)]
	[TestCase("maui-blazor", DotNetCurrent, "Debug", null)]
	[TestCase("maui-blazor", DotNetCurrent, "Release", null)]
	[TestCase("maui-blazor", DotNetCurrent, "Release", "full")]
	public void RunOnAndroid(string id, string framework, string config, string trimMode)

This slipped through, unfortunately, as we had missed solving all the trimmer warnings in Mono.Android.Export.dll! dotnet/android#9300 aims to fix that.

After dotnet/android#9300, new trimming warnings specific to .NET MAUI will surface such as the one above.

But we can easily replace [Export] by:

• Define a Java interface & create a binding for it in C#, we already have maui.aar setup for this.

• We can simply implement the interface in C# and remove [Export].

Lastly, I fixed some of the defaults in Metadata.xml, it didn't look like we were automatically making Java interfaces internal. It looks like we probably made ImageLoaderCallback public by mistake.

[jonathanpeppers]

The on-device HybridWebView tests look like they are still working on Android:

SendMessage() appears to still be called appropriately. 👍