Run ErrorProne in OpenJDK9

Use the OpenJDK9 javac to run ErrorProne by adding it to
-processorpath and using -Xplugin:ErrorProne.

Bug: 69485063
Test: m RUN_ERROR_PRONE=true javac-check
Change-Id: I0496006b71b70766ef16d57753cbcf037897799c
diff --git a/android/config.go b/android/config.go
index f5a5e8e..b415443 100644
--- a/android/config.go
+++ b/android/config.go
@@ -322,10 +322,8 @@
 func (c *config) fromEnv() error {
 	switch c.Getenv("EXPERIMENTAL_USE_OPENJDK9") {
 	case "":
-		if c.Getenv("RUN_ERROR_PRONE") != "true" {
-			// Use OpenJDK9, but target 1.8
-			c.useOpenJDK9 = true
-		}
+		// Use OpenJDK9, but target 1.8
+		c.useOpenJDK9 = true
 	case "false":
 		// Use OpenJDK8
 	case "1.8":
@@ -631,6 +629,10 @@
 	return Bool(c.productVariables.UseGoma)
 }
 
+func (c *config) RunErrorProne() bool {
+	return c.IsEnvTrue("RUN_ERROR_PRONE")
+}
+
 // Returns true if OpenJDK9 prebuilts are being used
 func (c *config) UseOpenJDK9() bool {
 	return c.useOpenJDK9