Support EXPERIMENTAL_USE_OPENJDK9=false.

This fourth possible value currently has the same semantics
as a default/unset EXPERIMENTAL_USE_OPENJDK9, but allows
people to explicitly switch back to the old semantics when
the default changes.

Test: make showcommands core-oj (in environments with
      EXPERIMENTAL_USE_OPENJDK9 set to "", "1.8", and "true").
Bug: 38177295

Change-Id: I25accf14344a05349a6e97572d7c2c1f6a7f2063