NIT lint warns fixed in BlobProvider (#39614)

Summary:
Pull Request resolved: #39614

NIT lint warns fixed in BlobProvider

changelog: [internal] internal

Reviewed By: christophpurrer

Differential Revision: D49483635

fbshipit-source-id: e097a8575c6e53d7270d200cdff11988c5f63356

Author: mdvacca

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/modules/blob/BlobProvider.java

@@ -27,7 +27,7 @@ public final class BlobProvider extends ContentProvider {
 
   private static final int PIPE_CAPACITY = 65536;
 
-  private ExecutorService executor = Executors.newSingleThreadExecutor();
+  private final ExecutorService executor = Executors.newSingleThreadExecutor();
 
   @Override
   public boolean onCreate() {
@@ -61,7 +61,8 @@ public int update(Uri uri, ContentValues values, String selection, String[] sele
   }
 
   @Override
-  public ParcelFileDescriptor openFile(Uri uri, String mode) throws FileNotFoundException {
+  public @Nullable ParcelFileDescriptor openFile(Uri uri, String mode)
+      throws FileNotFoundException {
     if (!mode.equals("r")) {
       throw new FileNotFoundException("Cannot open " + uri.toString() + " in mode '" + mode + "'");
     }
@@ -80,7 +81,7 @@ public ParcelFileDescriptor openFile(Uri uri, String mode) throws FileNotFoundEx
 
     final byte[] data = blobModule.resolve(uri);
     if (data == null) {
-      throw new FileNotFoundException("Cannot open " + uri.toString() + ", blob not found.");
+      throw new FileNotFoundException("Cannot open " + uri + ", blob not found.");
     }
 
     ParcelFileDescriptor[] pipe;