Add a droiddoc generator for hwbinder.stubs

hwbinder.stubs provides stubs for @SystemAPI hwbinder classes
that doesn't depend on framework, used to break a dependency cycle
between hidl interfaces and android_system_stubs_current while
satisifying the link type checks.

Bug: 77285514
Test: m checkbuild
Change-Id: Id4093e19f607dc5b0c663e1b637012799ab110c3