Group encoder algorithm config in AV1EncoderConfig

This CL groups the configuration parameters related to encoder
algorithm in AV1EncoderConfig into a new struct AlgoCfg, cleans
up function interfaces, and adds relevant documentation.

BUG=aomedia:2701

Change-Id: I7408066a2e2d34ccf1e27d6cacd464a30335b50e
diff --git a/av1/encoder/encoder.c b/av1/encoder/encoder.c
index cc6dd30..dd79d6b 100644
--- a/av1/encoder/encoder.c
+++ b/av1/encoder/encoder.c
@@ -2871,7 +2871,7 @@
     }
   }
 
-  switch (oxcf->cdf_update_mode) {
+  switch (oxcf->algo_cfg.cdf_update_mode) {
     case 0:  // No CDF update for any frames(4~6% compression loss).
       features->disable_cdf_update = 1;
       break;