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