Generate empty classpaths.proto for bootclasspath_fragment.go.

- Adds all required details for bootclasspath_fragment to implement
  classpath_fragment.
- Keeps the actual boot jars in platform-bootclasspath to begin with.
- Makes sure to put the file in apex/etc/classpath on device. Note that
  for platform versions of classpath fragment AndroidMkEntries perform
  the installation, while for APEXes it must be plumbed via apex.go.

Bug: 180105615
Test: m && launch_cvd; atest CtsClasspathsTestCases
Change-Id: I6101ebdf5b8bcbe95c0b7ce21f3f67a2685aef50
diff --git a/java/platform_bootclasspath.go b/java/platform_bootclasspath.go
index 1caffe4..c787e47 100644
--- a/java/platform_bootclasspath.go
+++ b/java/platform_bootclasspath.go
@@ -203,6 +203,7 @@
 func (b *platformBootclasspathModule) generateClasspathProtoBuildActions(ctx android.ModuleContext) {
 	// ART and platform boot jars must have a corresponding entry in DEX2OATBOOTCLASSPATH
 	classpathJars := configuredJarListToClasspathJars(ctx, b.ClasspathFragmentToConfiguredJarList(ctx), BOOTCLASSPATH, DEX2OATBOOTCLASSPATH)
+
 	// TODO(satayev): remove updatable boot jars once each apex has its own fragment
 	global := dexpreopt.GetGlobalConfig(ctx)
 	classpathJars = append(classpathJars, configuredJarListToClasspathJars(ctx, global.UpdatableBootJars, BOOTCLASSPATH)...)