Use USE_OPENJDK9 instead of EXPERIMENTAL_USE_OPENJDK9.

This avoids duplicating the semantics of an unset/empty
EXPERIMENTAL_USE_OPENJDK9.

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

Change-Id: Ibcd85c36a2fee78071ed382a81df0f9634e6ac8f
1 file changed