Enable nullable reference types in AndroidDiskInformation.cs (#11813)

`src/Mono.AndroidTools/AndroidDiskInformation.cs` ships in the signed `Mono.AndroidTools` assembly but was nullable-oblivious. This opts the file into nullable reference types for compile-time null-safety with no runtime or API change.

### Changes
- Added `#nullable enable` to the top of the file.
- Changed the `s` locals in `ParseNewFormat` and `ParseOldFormat` from `string` to `string?`, matching `StringReader.ReadLine()`'s `string?` return. Flow analysis already narrows `s` to non-null inside the `while ((s = reader.ReadLine ()) != null)` bodies.

```csharp
string? s;
var reader = new StringReader (output);
while ((s = reader.ReadLine ()) != null) {
    // s is non-null here
}
```

Annotation-only: no `!` operator, no `ArgumentNullException.ThrowIfNull` (the project targets `netstandard2.0`), and public signatures (`FromDfOutput`, `InternalSpace`, `ExternalSpace`) are unchanged.

Author: Copilot

src/Mono.AndroidTools/AndroidDiskInformation.cs

@@ -26,6 +26,7 @@
 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 // THE SOFTWARE.
 
+#nullable enable
 using System;
 using System.IO;
 
@@ -67,7 +68,7 @@ public static AndroidDiskInformation FromDfOutput (string dfOutput)
 
 		void ParseNewFormat (string output)
 		{
-			string s;
+			string? s;
 			var reader = new StringReader (output);
 
 			reader.ReadLine (); //  header line
@@ -95,7 +96,7 @@ void ParseNewFormat (string output)
 
 		void ParseOldFormat (string output)
 		{
-			string s;
+			string? s;
 			var reader = new StringReader (output);
 			while ((s = reader.ReadLine ()) != null) {
 				int idx = s.IndexOf (':');