Move error_prone config to external/error_prone

The error_prone config will likely need to be updated at the same
time as the error_prone prebuilts, so move the config into the same
project.  This requires jumping through some hoops because
external/error_prone may not be present in all manifests.

Bug: 64489631
Test: m -j RUN_ERROR_PRONE=true
Change-Id: I2c59df1148134ffedac0e00d32ac7082a9fb5330
diff --git a/java/config/config.go b/java/config/config.go
index 4a5a2dd..047c4cb 100644
--- a/java/config/config.go
+++ b/java/config/config.go
@@ -26,15 +26,14 @@
 var (
 	pctx = android.NewPackageContext("android/soong/java/config")
 
-	JavacHeapSize = "2048M"
-
 	DefaultLibraries = []string{"core-oj", "core-libart", "ext", "framework", "okhttp"}
 )
 
 func init() {
 	pctx.Import("github.com/google/blueprint/bootstrap")
 
-	pctx.StaticVariable("JavacHeapFlags", "-J-Xmx"+JavacHeapSize)
+	pctx.StaticVariable("JavacHeapSize", "2048M")
+	pctx.StaticVariable("JavacHeapFlags", "-J-Xmx${JavacHeapSize}")
 
 	pctx.StaticVariable("CommonJdkFlags", strings.Join([]string{
 		`-Xmaxerrs 9999999`,