[xabt] dotnet watch support, based on env vars (#10960)

Context: dotnet/sdk#52492
Context: dotnet/sdk#52581

`dotnet-watch` now runs Android applications via:

    dotnet watch 🚀 [helloandroid (net10.0-android)] Launched 'D:\src\xamarin-android\bin\Debug\dotnet\dotnet.exe' with arguments 'run --no-build -e DOTNET_WATCH=1 -e DOTNET_WATCH_ITERATION=1 -e DOTNET_MODIFIABLE_ASSEMBLIES=debug -e DOTNET_WATCH_HOTRELOAD_WEBSOCKET_ENDPOINT=ws://localhost:9000 -e DOTNET_STARTUP_HOOKS=D:\src\xamarin-android\bin\Debug\dotnet\sdk\10.0.300-dev\DotnetTools\dotnet-watch\10.0.300-dev\tools\net10.0\any\hotreload\net10.0\Microsoft.Extensions.DotNetDeltaApplier.dll -bl': process id 3356

And so the pieces on Android for this to work are:

~~ Startup Hook Assembly ~~

Parse out the value:

    <_AndroidHotReloadAgentAssemblyPath>@(RuntimeEnvironmentVariable->WithMetadataValue('Identity', 'DOTNET_STARTUP_HOOKS')->'%(Value)'->Exists())</_AndroidHotReloadAgentAssemblyPath>

And verify this assembly is included in the app:

    <ResolvedFileToPublish Include="$(_AndroidHotReloadAgentAssemblyPath)" />

Then, for Android, we need to patch up `$DOTNET_STARTUP_HOOKS` to be
just the assembly name, not the full path:

    <_AndroidHotReloadAgentAssemblyName>$([System.IO.Path]::GetFileNameWithoutExtension('$(_AndroidHotReloadAgentAssemblyPath)'))</_AndroidHotReloadAgentAssemblyName>
    ...
    <RuntimeEnvironmentVariable Include="DOTNET_STARTUP_HOOKS" Value="$(_AndroidHotReloadAgentAssemblyName)" />

~~ Port Forwarding ~~

A new `_AndroidConfigureAdbReverse` target runs after deploying apps,
that does:

    adb reverse tcp:9000 tcp:9000

I parsed the value out of:

    <_AndroidWebSocketEndpoint>@(RuntimeEnvironmentVariable->WithMetadataValue('Identity', 'DOTNET_WATCH_HOTRELOAD_WEBSOCKET_ENDPOINT')->'%(Value)')</_AndroidWebSocketEndpoint>
    <_AndroidWebSocketPort>$([System.Text.RegularExpressions.Regex]::Match('$(_AndroidWebSocketEndpoint)', ':(\d+)').Groups[1].Value)</_AndroidWebSocketPort>

~~ Prevent Startup Hooks in Microsoft.Android.Run ~~

When I was implementing this, I keep seeing *two* clients connect to
`dotnet-watch` and I was pulling my hair to figure out why!

Then I realized that `Microsoft.Android.Run` was also getting
`$DOTNET_STARTUP_HOOKS`, and so we had a desktop process + mobile
process both trying to connect!

Easiest fix, is to disable startup hook support in
`Microsoft.Android.Run`. I reviewed the code in `dotnet run`, and it
doesn't seem correct to try to clear the env vars.

~~ Conclusion ~~

With these changes, everything is working!

    dotnet watch 🔥 C# and Razor changes applied in 23ms.

* Include $(AdbTarget) in adb reverse command

When multiple devices/emulators are connected, the adb reverse
command needs  (e.g. -s emulator-5554) to target
the correct device. Without it, the command fails with
'more than one device' or forwards on the wrong device.

NOTE: we run the test on MonoVM-only for now, until we get:
* dotnet/sdk#53501

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: jonathanpeppers

src/Xamarin.Android.Build.Tasks/Microsoft.Android.Sdk/targets/Microsoft.Android.Sdk.HotReload.targets

@@ -76,7 +76,7 @@ See: https://github.com/dotnet/sdk/pull/52581
     </PropertyGroup>
 
     <Exec Condition=" '$(_AndroidWebSocketPort)' != '' "
-          Command="&quot;$(_AdbToolPath)&quot; reverse tcp:$(_AndroidWebSocketPort) tcp:$(_AndroidWebSocketPort)" />
+          Command="&quot;$(_AdbToolPath)&quot; $(AdbTarget) reverse tcp:$(_AndroidWebSocketPort) tcp:$(_AndroidWebSocketPort)" />
 
   </Target>