android_jni: Set threads to 2 instead of CPU count

Empirically, on Android devices with more than 1 core, it is
almost always better to use 2 threads than to use
number_of_cpu_cores threads.

Update the public java API function documentation to reflect this
change.

The new behavior is:
* negative values map to number of cpu cores.
* zero maps to 1 or 2 depending on single core or multi core devices.
* value >=0 maps to value.

This change ensures that users who use the library via JNI take the
optimal path when using the default value of 0.

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

@@ -110,10 +110,11 @@ public static boolean decode(ByteBuffer encoded, int length, Bitmap bitmap) {
    * @param bitmap The decoded pixels will be copied into the bitmap.
    *     If the bitmap dimensions do not match the decoded image's dimensions,
    *               then the decoded image will be scaled to match the bitmap's dimensions.
-   * @param threads Number of threads to be used for the AVIF decode. Zero means use number of CPU
-   *     cores as the thread count. Negative values are invalid. When this value is > 0, it is
-   *     simply mapped to the maxThreads parameter in libavif. For more details, see the
-   *     documentation for maxThreads variable in avif.h.
+   * @param threads Number of threads to be used for the AVIF decode. Zero means use the library
+   *     determined optimal value as the thread count. Negative values mean use number of CPU cores
+   *     as the thread count. When this value is > 0, it is simply mapped to the maxThreads
+   *     parameter in libavif. For more details, see the documentation for maxThreads variable in
+   *     avif.h.
    * @return true on success and false on failure. A few possible reasons for failure are: 1) Input
    *     was not valid AVIF. 2) Negative value was passed for the threads parameter.
    */

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

@@ -6,6 +6,7 @@
 #include <cpu-features.h>
 #include <jni.h>
 
+#include <algorithm>
 #include <cstdio>
 #include <memory>
 #include <new>
@@ -210,7 +211,15 @@ avifResult DecodeNthImage(JNIEnv* const env, AvifDecoderWrapper* const decoder,
 }
 
 int getThreadCount(int threads) {
-  return (threads == 0) ? android_getCpuCount() : threads;
+  if (threads < 0) {
+    return android_getCpuCount();
+  }
+  if (threads == 0) {
+    // Empirically, on Android devices with more than 1 core, decoding with 2
+    // threads is almost always better than using as many threads as CPU cores.
+    return std::min(android_getCpuCount(), 2);
+  }
+  return threads;
 }
 
 // Checks if there is a pending JNI exception that will be thrown when the