Create a new mode in soong_ui to generate API only BUILD files

The generated Bazel workspace will only contain api specific targets.
This is feasible since these targets do not have any cross dependencies
with the targets in the bp2build workspace

The advantages of a new mode are
1. Does not pollute bp2build workspace with api targets
2. Does not block api targets with the current allowlist conversion
   mechansims in bp2build
(In the future we might want to combine these two workspaces)

A Soong module type will generate a Bazel target if it implements
ApiProvider interface

Test: m apigen
Test: m nothing

Change-Id: I69c57ca6539f932e0ad554ce84a87fb7936fdba0