Revert^2 "Generate a boot image extension for mainline BCP...

Revert submission 2465993-boot-image-mainline-revert

Reason for revert: Relanding the changes

Reverted changes: /q/submissionid:2465993-boot-image-mainline-revert

Change-Id: Ie611cf3a1ff4be7a7beab164f69b4186c474be5c
diff --git a/java/platform_bootclasspath.go b/java/platform_bootclasspath.go
index 5824f08..0ea3609 100644
--- a/java/platform_bootclasspath.go
+++ b/java/platform_bootclasspath.go
@@ -414,6 +414,7 @@
 	frameworkBootImageConfig := defaultBootImageConfig(ctx)
 	bootFrameworkProfileRule(ctx, frameworkBootImageConfig)
 	b.generateBootImage(ctx, frameworkBootImageName, platformModules)
+	b.generateBootImage(ctx, mainlineBootImageName, apexModules)
 	b.copyApexBootJarsForAppsDexpreopt(ctx, apexModules)
 	dumpOatRules(ctx, frameworkBootImageConfig)
 }