Update ANDROID_NDK_API_LEVEL default in configure.py

PiperOrigin-RevId: 506335783

Author: terryheo

configure.py

@@ -757,7 +757,7 @@ def valid_api_level(api_level):
   android_ndk_api_level = prompt_loop_or_load_from_env(
       environ_cp,
       var_name='ANDROID_NDK_API_LEVEL',
-      var_default='21',  # 21 is required for ARM64 support.
+      var_default='26',  # 26 is required to support AHardwareBuffer.
       ask_for_var=('Please specify the (min) Android NDK API level to use. '
                    '[Available levels: %s]') % api_levels,
       check_success=valid_api_level,