[non-normative] Set the default number of encoder threads to 1

Set the default number of threads to 1 in the encoder to make it
less confusing in testing.

Cherry picked https://aomedia-review.googlesource.com/c/aom/+/65461
from master branch, which was needed to resolve issue 2278.

BUG=aomedia:2278

Change-Id: I0c782abee7145b1121abc2ae495b3b25b25e451e

av1/av1_cx_iface.c

diff --git a/av1/av1_cx_iface.c b/av1/av1_cx_iface.c
index 9d5414c1..3fec008 100644
--- a/av1/av1_cx_iface.c
+++ b/av1/av1_cx_iface.c
@@ -1728,7 +1728,7 @@
     {
         // NOLINT
         0,  // g_usage
-        8,  // g_threads
+        0,  // g_threads
         0,  // g_profile
 
         320,         // g_width