Build with java7 by default.

All introduce a flag LEGACY_USE_JAVA6 to force java6 builds.
This is an unsupported configuration, and provided temporarily
to iron out regressions and compare build output (if required.).

- Increment the version check sequence number.
- Move a more specific check (OpenJDK vs non OpenJDK) after
  the more general version check.
- Update the link in the version check error message to the
  "initializing" page instead of the "download" page. The latter
  talks about repo, mainly.

bug: 8992787

Change-Id: I313e17b1911768d4f3bc318c4162c53dec6eaf0d

Conflicts:
	core/main.mk
diff --git a/core/combo/javac.mk b/core/combo/javac.mk
index 8c24e4b..528e556 100644
--- a/core/combo/javac.mk
+++ b/core/combo/javac.mk
@@ -9,7 +9,7 @@
 #   COMMON_JAVAC -- Java compiler command with common arguments
 #
 
-ifeq ($(EXPERIMENTAL_USE_JAVA7),)
+ifneq ($(LEGACY_USE_JAVA6),)
 common_flags := -target 1.5 -Xmaxerrs 9999999
 else
 common_flags := -source 1.7 -target 1.7 -Xmaxerrs 9999999