Fix isHeif crash (#177944)

Fixes flutter/flutter#177942

I did not repro the crash but this looks like a pretty simple NPE that
can be fixed by swapping the `.equals` order.

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---------

Co-authored-by: Gray Mackall <[email protected]>

Author: gmackall

engine/src/flutter/shell/platform/android/io/flutter/embedding/engine/image/Metadata.java

@@ -5,6 +5,7 @@
 package io.flutter.embedding.engine.image;
 
 import androidx.annotation.NonNull;
+import androidx.annotation.Nullable;
 import androidx.annotation.RequiresApi;
 import androidx.annotation.VisibleForTesting;
 import io.flutter.Build;
@@ -20,7 +21,7 @@ class Metadata {
   int width;
   int height;
   int rotation;
-  String mimeType;
+  @Nullable String mimeType;
   int orientation;
   int originalHeight;
   int originalWidth;
@@ -52,6 +53,6 @@ static Metadata create(
    * @return True if the image is HEIF, false otherwise.
    */
   boolean isHeif() {
-    return mimeType.equals("image/heif");
+    return "image/heif".equals(mimeType);
   }
 }

engine/src/flutter/shell/platform/android/test/io/flutter/embedding/engine/image/MetadataTest.java

@@ -151,4 +151,11 @@ public void isHeif_returnsCorrectValue() {
       // Expected
     }
   }
+
+  @Test
+  public void isHeif_doesNotCrashWithNullMetatdata() {
+    Metadata metadata = new Metadata();
+    metadata.mimeType = null;
+    assertFalse(metadata.isHeif());
+  }
 }