Add experimental flag to support OpenJDK builds.

bug: 8992787

(cherry picked from commit d1dbe7f5a23bacdff086b4992e1d31a96da706c3)

Change-Id: Ie977d4f26ae8d3eee9d7b9cf2c0c270bee5b54b3

Conflicts:
	core/combo/javac.mk
	core/main.mk
A few more Java7 related clean ups.

- Separate SDK checking from version checking and
  make messages clearer.
- Add explicit source & target versions for javac to
  make things clearer.
- Rename flag from EXPERIMENTAL_USE_JAVA7_OPENJDK to
  EXPERIMENTAL_USE_JAVA7.
- Allow Oracle JDK 1.7 to be used on Mac OS, since there's
  no official OpenJDK support for that platform.

Change-Id: I454d2c917ed78f03ec7559a99659fefe7e7d50f3
Add support for ALTERNATE_JAVAC

Bug: 12476356
Change-Id: Idd013031dc32a560bca76cca295bedcad55982b4
Increase the maximum javac heap size.

Java 7 needs more memory when you run some annotation tools.

Change-Id: Ia8b423244b9ce164500b24867fe2b3d10a5089ff
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

Conflicts:
	core/config.mk
	core/main.mk
	envsetup.sh
Dont stop if using Oracle on linux, only give a warning.