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/android/config.go b/android/config.go
index 3db7980..79917ad 100644
--- a/android/config.go
+++ b/android/config.go
@@ -1564,6 +1564,15 @@
 	return false
 }
 
+// ApexOfJar returns the apex component of the first pair with the given jar name on the list, or
+// an empty string if not found.
+func (l *ConfiguredJarList) ApexOfJar(jar string) string {
+	if idx := IndexList(jar, l.jars); idx != -1 {
+		return l.Apex(IndexList(jar, l.jars))
+	}
+	return ""
+}
+
 // IndexOfJar returns the first pair with the given jar name on the list, or -1
 // if not found.
 func (l *ConfiguredJarList) IndexOfJar(jar string) int {