[Mono.AndroidTools] Enable nullable reference types in AdbSyncNotification.cs (#11778)

`AdbSyncNotification.cs` ships in `Mono.AndroidTools.dll` but compiled in a nullable-oblivious context. This opts the file into NRT, making its existing null contract explicit: `LocalPath` is `null` for directory/remove/preserve operations, while `RemotePath` is always non-null.

## Changes
- **`src/Mono.AndroidTools/Adb/AdbSyncNotification.cs`**
  - Added `#nullable enable` as the first line.
  - Annotated `LocalPath` property and the `localPath` constructor parameter as `string?`; `RemotePath` stays non-nullable.

```csharp
public AdbSyncNotification (AdbSyncKind kind, string remotePath, string? localPath, long size)
...
public string? LocalPath { get; private set; }
```

No null-throwing is required (`netstandard2.0`, so no `ArgumentNullException.ThrowIfNull`), and callers in `AdbSyncContext.cs`, `AdbSyncClient.cs`, and `AndroidDevice.cs` are not nullable-enabled, so no call-site changes or warnings result.

Author: Copilot

src/Mono.AndroidTools/Adb/AdbSyncNotification.cs

@@ -1,3 +1,4 @@
+#nullable enable
 // 
 // AdbSyncNotification.cs
 //  
@@ -33,7 +34,7 @@ namespace Mono.AndroidTools.Adb
 	/// </summary>
 	public class AdbSyncNotification
 	{
-		public AdbSyncNotification (AdbSyncKind kind, string remotePath, string localPath, long size)
+		public AdbSyncNotification (AdbSyncKind kind, string remotePath, string? localPath, long size)
 		{
 			this.Kind = kind;
 			this.RemotePath = remotePath;
@@ -43,7 +44,7 @@ public AdbSyncNotification (AdbSyncKind kind, string remotePath, string localPat
 
 		public AdbSyncKind Kind { get; private set; }
 		public string RemotePath { get; private set; }
-		public string LocalPath { get; private set; }
+		public string? LocalPath { get; private set; }
 		public long Size { get; private set; }
 
 		public string GetMessage ()