android_jni: Return NOT_IMPLEMENTED for invalid bitmap

Also add a comment for the threads parameter.

GOOGLE_INTERNAL_CL: 526057315

android_jni/avifandroidjni/src/main/java/org/aomedia/avif/android/AvifDecoder.java

diff --git a/android_jni/avifandroidjni/src/main/java/org/aomedia/avif/android/AvifDecoder.java b/android_jni/avifandroidjni/src/main/java/org/aomedia/avif/android/AvifDecoder.java
index 773720a..4cb1716 100644
--- a/android_jni/avifandroidjni/src/main/java/org/aomedia/avif/android/AvifDecoder.java
+++ b/android_jni/avifandroidjni/src/main/java/org/aomedia/avif/android/AvifDecoder.java
@@ -171,7 +171,7 @@
    */
   @Nullable
   public static AvifDecoder create(ByteBuffer encoded) {
-    return create(encoded, 1);
+    return create(encoded, /* threads= */ 1);
   }
 
   /**

android_jni/avifandroidjni/src/main/jni/libavif_jni.cc

diff --git a/android_jni/avifandroidjni/src/main/jni/libavif_jni.cc b/android_jni/avifandroidjni/src/main/jni/libavif_jni.cc
index 16bba1a..01c6181 100644
--- a/android_jni/avifandroidjni/src/main/jni/libavif_jni.cc
+++ b/android_jni/avifandroidjni/src/main/jni/libavif_jni.cc
@@ -100,7 +100,7 @@
       bitmap_info.format != ANDROID_BITMAP_FORMAT_RGB_565 &&
       bitmap_info.format != ANDROID_BITMAP_FORMAT_RGBA_F16) {
     LOGE("Bitmap format (%d) is not supported.", bitmap_info.format);
-    return AVIF_RESULT_UNKNOWN_ERROR;
+    return AVIF_RESULT_NOT_IMPLEMENTED;
   }
   void* bitmap_pixels = nullptr;
   if (AndroidBitmap_lockPixels(env, bitmap, &bitmap_pixels) !=