Add comment to File copy on linux/android.

runtime/bin/file_android.cc

Index: runtime/bin/file_android.cc
diff --git a/runtime/bin/file_android.cc b/runtime/bin/file_android.cc
index e253417065828ccdc13062742df61170a9003965..0176ba632fa4f6571e14c14c937161ab172f0c8b 100644
--- a/runtime/bin/file_android.cc
+++ b/runtime/bin/file_android.cc
@@ -244,8 +244,11 @@ bool File::Copy(const char* old_path, const char* new_path) {
       result = TEMP_FAILURE_RETRY(
           sendfile(new_fd, old_fd, &offset, kMaxUint32));
     }
+    // From sendfile man pages:
+    //   Applications may wish to fall back to read(2)/write(2) in the case
+    //   where sendfile() fails with EINVAL or ENOSYS.
     if (result < 0 && (errno == EINVAL || errno == ENOSYS)) {
-      const intptr_t kBufferSize = 8 * 1024;
+      const intptr_t kBufferSize = 8 * KB;
       uint8_t buffer[kBufferSize];
       while ((result = TEMP_FAILURE_RETRY(
           read(old_fd, buffer, kBufferSize))) > 0) {