Generate an api-versions.xml from stubs jar

The current api-versions.xml shipped in the SDK and used internally in
the android build system is generated by the framework-doc-stubs target.

This CL adds a new target that generates an (almost) equivalent database
in a different way. There are a few upshots of using this method to
generate them:
- using the framework-doc-stubs target is inefficient. It runs late in
  the build but also takes a long time, extending the critical path of
  the modules that depend on api-versions.xml
- the framework-doc-system-stubs target is using module sources as
  input, which is desireable to avoid because we want to separate
  module sources from the rest of the platform.

Bug: 187398174
Bug: 186197911
Test: m api_versions_{public,system} && diff with "originals"
Change-Id: I8adab94ee7ca0bee4fc0be8d99f1125360387d89