Split droidstubs out of droiddoc.go

Split part of droiddoc.go into droidstubs.go.  Also split droiddoc_test.go
and droidstubs_test.go out of java_test.go.

Test: go test ./java
Change-Id: Iea742e75b6925b135016f7bbf3a168c696a6c433

java/Android.bp

@@ -38,6 +38,7 @@ bootstrap_go_package {
         "dexpreopt_bootjars.go",
         "dexpreopt_config.go",
         "droiddoc.go",
+        "droidstubs.go",
         "gen.go",
         "genrule.go",
         "hiddenapi.go",
@@ -72,6 +73,8 @@ bootstrap_go_package {
         "device_host_converter_test.go",
         "dexpreopt_test.go",
         "dexpreopt_bootjars_test.go",
+        "droiddoc_test.go",
+        "droidstubs_test.go",
         "hiddenapi_singleton_test.go",
         "java_test.go",
         "jdeps_test.go",