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/apex/apex.go b/apex/apex.go
index 949d80e..7ceb00b 100644
--- a/apex/apex.go
+++ b/apex/apex.go
@@ -2097,6 +2097,11 @@
 		}
 	}
 
+	// Add classpaths.proto config.
+	classpathProtoOutput := bootclasspathFragmentInfo.ClasspathFragmentProtoOutput
+	classpathProto := newApexFile(ctx, classpathProtoOutput, classpathProtoOutput.Base(), bootclasspathFragmentInfo.ClasspathFragmentProtoInstallDir.Rel(), etc, nil)
+	filesToAdd = append(filesToAdd, classpathProto)
+
 	return filesToAdd
 }