Revert "darwin: Improve JAVA_HOME assumptions"

This reverts commit ca1be5ac94065dc0531ebbdf60d644c821ddcf77.
Make envsetup.sh set JAVA_HOME correctly for java7.

Note that despite the location of this change, this function
is run only during "lunch" and not during ". build/envsetup.sh"

Also, make it easier to switch back and forth between java6 and 7
on the same session.

bug: 8992787

Change-Id: I56ec0ba8552b46c04204a8b96b9abc0180f7605f

Conflicts:
	envsetup.sh
darwin: Improve JAVA_HOME assumptions

In the case of installing oracle's jdk package, jdk home may be in
a path such as /Library/Java/JavaVirtualMachines/jdk1.7.0_51.jdk/Home.
In this case, /usr/libexec/java_home reports the correct path even when
$JAVA_HOME is unset.

If JAVA_HOME is unset on darwin, check for the existence of the
/usr/libexec/java_home utility. If it points to the requested java major
version, then use it to set JAVA_HOME. Otherwise, set the fallback path.

Change-Id: Id698078d2fc8d6d258f87ebee211f49882c4dc56