Generate a boot image extension for mainline BCP jars.

Bug: 269230245
Test: m
Test: atest art_standalone_dexpreopt_tests
Change-Id: I253c30d938eee2cf2549ec2338425d53956e7cbb
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)
 }