Package releasetools.py in target_files.zip

This CL creates an intermediate filegroup, and adds that to
`Releasetools_extension` dep of the autogenerated android_device. This
dep will be used for packaging target_files.zip

Bug: 3493530
Test: Built and diff'd target_files.zip
Change-Id: I9a5a2c3dba5fd37924544e92cb2646837047600a
diff --git a/android/variable.go b/android/variable.go
index 4e1df3c..81999f3 100644
--- a/android/variable.go
+++ b/android/variable.go
@@ -713,6 +713,8 @@
 	ProductFsCasefold    string `json:",omitempty"`
 	ProductQuotaProjid   string `json:",omitempty"`
 	ProductFsCompression string `json:",omitempty"`
+
+	ReleaseToolsExtensionDir string `json:",omitempty"`
 }
 
 func boolPtr(v bool) *bool {