Add Android.bp.list and soong.variables to inputs for soong_docs action

While this is not a comprehensive list of all inputs of this action (as the action
depends on all blueprint files in the source tree), this closer to the truth.

Test: Manually verified ninja output, checkbuild approved validation for aosp-crosshatch
Change-Id: I77a0f612afb025232bee7d3ac31257b808991829