Enable payload v2.

Switch the preferred payload major version to 2.

Bug: 23694580
Test: Deployed signed and unsigned payload to a device.

Change-Id: Ib338883510c1c8ddaad3e4c77a769799ee058b64
diff --git a/update_engine.conf b/update_engine.conf
index dea31e4..981887d 100644
--- a/update_engine.conf
+++ b/update_engine.conf
@@ -1,2 +1,2 @@
-PAYLOAD_MAJOR_VERSION=1
+PAYLOAD_MAJOR_VERSION=2
 PAYLOAD_MINOR_VERSION=2