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

java/classpath_fragment.go

@@ -143,6 +143,8 @@ func writeClasspathsJson(ctx android.ModuleContext, output android.WritablePath,
 	android.WriteFileRule(ctx, output, content.String())
 }
 
+// Returns AndroidMkEntries objects to install generated classpath.proto.
+// Do not use this to install into APEXes as the injection of the generated files happen separately for APEXes.
 func (c *ClasspathFragmentBase) androidMkEntries() []android.AndroidMkEntries {
 	return []android.AndroidMkEntries{{
 		Class:      "ETC",