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