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