Refine adaptive_txb_search_level speed feature for speed 4

For speed = 4 preset, BD-rate impact is seen as 0.02% (as per AWCY runs),
with encode time reduction of 1.07% (averaged across multiple test cases).

STATS_CHANGED

Change-Id: I21c716b9ddc6a6dae8fd10db94de73e91d833c93
diff --git a/av1/encoder/speed_features.c b/av1/encoder/speed_features.c
index 1916dcc..25c3c72 100644
--- a/av1/encoder/speed_features.c
+++ b/av1/encoder/speed_features.c
@@ -349,6 +349,7 @@
     sf->alt_ref_search_fp = 1;
     sf->skip_sharp_interp_filter_search = 1;
     sf->perform_coeff_opt = boosted ? 0 : 4;
+    sf->adaptive_txb_search_level = boosted ? 2 : 3;
   }
 
   if (speed >= 5) {