Disable libaom decoder by default

To unblock roll of newer build files for libaom that are using
differen build flags to include/exclude libaom

Bug: webrtc:11404
Change-Id: If06b63e0835e65113617efa29f34ba6bb309c16d
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/170630
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#30810}

.gn

@@ -72,7 +72,7 @@ default_args = {
   # for unittests, it can be disabled (see third_party/libyuv/BUILD.gn)
   libyuv_use_gflags = false
 
-  enable_libaom_decoder = true
+  enable_libaom_decoder = false
 
   gtest_enable_absl_printers = true
 }