Generalize the platformBootclasspathDepsMutator

Adds a BootclasspathDepsMutator that is currently only implemented by
the platform_bootclasspath but which can be implemented by
bootclasspath_fragment too.

Bug: 177892522
Test: m nothing
Change-Id: Ibe35854281004d6e40bf1f797144fb582e8c08b9
diff --git a/java/testing.go b/java/testing.go
index 08a71b8..e9cdddc 100644
--- a/java/testing.go
+++ b/java/testing.go
@@ -232,6 +232,7 @@
 	RegisterAppBuildComponents(ctx)
 	RegisterAppImportBuildComponents(ctx)
 	RegisterAppSetBuildComponents(ctx)
+	registerBootclasspathBuildComponents(ctx)
 	RegisterBootImageBuildComponents(ctx)
 	RegisterDexpreoptBootJarsComponents(ctx)
 	RegisterDocsBuildComponents(ctx)