Include the major version in update_engine.conf

This patch moves all the payload definition version numbers to
payload_constants.h and exposes the supported one in the .conf file.

Bug: 23946683
Test: Added unittest to match the .conf file with the code.

Change-Id: I7d84d2aa3c85d2b4d5da8bf102aa9bf99acc0136
diff --git a/update_engine.conf b/update_engine.conf
index dc65ebe..dea31e4 100644
--- a/update_engine.conf
+++ b/update_engine.conf
@@ -1 +1,2 @@
+PAYLOAD_MAJOR_VERSION=1
 PAYLOAD_MINOR_VERSION=2