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
diff --git a/ui/build/config.go b/ui/build/config.go
index 940bb2f..6e5559d 100644
--- a/ui/build/config.go
+++ b/ui/build/config.go
@@ -149,7 +149,7 @@
 		if override, ok := ret.environ.Get("OVERRIDE_ANDROID_JAVA_HOME"); ok {
 			return override
 		}
-		if v, ok := ret.environ.Get("EXPERIMENTAL_USE_OPENJDK9"); ok && v != "" {
+		if v, ok := ret.environ.Get("EXPERIMENTAL_USE_OPENJDK9"); ok && v != "" && v != "false" {
 			return filepath.Join("prebuilts/jdk/jdk9", ret.HostPrebuiltTag())
 		}
 		return filepath.Join("prebuilts/jdk/jdk8", ret.HostPrebuiltTag())