Moves generated android_sdk() and android_ndk() repo rules out of WORKSPACE.

These rules currently get written by configure.py script to WORKSPACE
file which is not ideal since (1) WORKSPACE file is tracked by git and
(2) we require users to manually delete the rules in order to
update/regenerate them.

Moving these rules into an external repo that is generated based on
several ENV variables set by the configure.py script. Modifying any
of these ENV variables will cause the rules to be updated.

PiperOrigin-RevId: 199388460