Add genrule for combined-removed-dex

This is a module that aims to have the same contents as
out/soong/hiddenapi/combined-dex-api generated by hiddenapi_singleton.go
inside soong, but generated using a different method that does not
depend on the monolithic droidstubs rule in frameworks/base.

This is part of a series of changes to remove all the dependencies on
the monolithic droidstub rules, so that they can eventually be removed.

Bug: 169034951
Bug: 169395887
Test: m combined-dex-api
Change-Id: I2c4a76aa1d85ef44fc467474afacec20901aee13