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