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

android/config.go

@@ -291,6 +291,8 @@ func (c *config) fromEnv() error {
 	switch c.Getenv("EXPERIMENTAL_USE_OPENJDK9") {
 	case "":
 		// Use OpenJDK8
+	case "false":
+		// Use OpenJDK8
 	case "1.8":
 		// Use OpenJDK9, but target 1.8
 		c.useOpenJDK9 = true