Final switch from Python 2 to Python 3 for avbtool.

As of January 1st, 2020 Python 2 is not officially supported anymore:
https://www.python.org/dev/peps/pep-0373/#id4

As a consequence avbtool has been ported to Python 3 over the last
couple of changes in a still Python 2 compatible way. This particular
change is the last step in the porting work to switch over to Python 3
and breaks backward compatibility with Python 2 runtime environments.

Bug: 151336743
Test: atest --host libavb_host_unittest
Test: atest --host aftltool_test
Test: ./aftltool_integration_test.py
Change-Id: I4a93a644c427c0ca9be70addd0f26d5e4ff1556e
diff --git a/Android.bp b/Android.bp
index d6597a2..1587fb5 100644
--- a/Android.bp
+++ b/Android.bp
@@ -81,11 +81,10 @@
     required: ["fec"],
     version: {
         py2: {
-            enabled: true,
-            embedded_launcher: true,
+            enabled: false,
         },
         py3: {
-            enabled: false,
+            enabled: true,
         },
     },
 }