Droiddoc Support in Soong

Support Droiddoc to Soong based on core/droiddoc.mk. The non-std doclet
based droiddoc compilation output is a "real" stubs.jar instead of a
directory of java files and a timestamp file.

The std doclet based javadoc compilation output is a "empty" stubs.jar
instead of a timestamp file.

The stubs.jar will be exported to
out/target/common/obj/JAVA_LIBRARIES/$(LOCAL_MODULE)_intermediates/classes.jar
and out/target/common/docs/$(LOCAL_MODULE)-stubs.jar

A $(LOCAL_MODULE).zip file will be generated also, and is exported to
out/target/common/docs/$(LOCAL_MODULE)-docs.zip if property: installable is not set
to false.

Bug: b/70351683
Test: unittest + convert libcore docs Android.mk to Soong manually.

Change-Id: I1cffddd138a5d9d445f86a3d4a3fd4de88a2bc0f
(cherry picked from commit 78188ec622cb1ee24171455867fc58ffab91562e)

java/builder.go

@@ -428,13 +428,6 @@ func (x *classpath) FormDesugarClasspath(optName string) []string {
 	return flags
 }
 
-// Append an android.Paths to the end of the classpath list
-func (x *classpath) AddPaths(paths android.Paths) {
-	for _, path := range paths {
-		*x = append(*x, path)
-	}
-}
-
 // Convert a classpath to an android.Paths
 func (x *classpath) Paths() android.Paths {
 	return append(android.Paths(nil), (*x)...)