Add missing soong_docs inputs

Specifically, make Android.bp.list and soong.variables explicit inputs

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 is closer to the truth.

This is a rollforward CL, which was originally rolled
back, as path validation failed for when OUT_DIR was
an absolute path. Validation has now been relaxed.

Test: Manually verified ninja output, checkbuild approved validation
  for aosp-crosshatch, and manually ran frameworks/rs/build_rs.py with
  patch fix
Change-Id: I4eb0d517f57336dd54eaa4bd31f46df9e93e6da2

android/writedocs.go

@@ -44,6 +44,10 @@ func primaryBuilderPath(ctx SingletonContext) Path {
 }
 
 func (c *docsSingleton) GenerateBuildActions(ctx SingletonContext) {
+	var deps Paths
+	deps = append(deps, pathForBuildToolDep(ctx, ctx.Config().moduleListFile))
+	deps = append(deps, pathForBuildToolDep(ctx, ctx.Config().ProductVariablesFileName))
+
 	// Generate build system docs for the primary builder.  Generating docs reads the source
 	// files used to build the primary builder, but that dependency will be picked up through
 	// the dependency on the primary builder itself.  There are no dependencies on the
@@ -63,6 +67,7 @@ func (c *docsSingleton) GenerateBuildActions(ctx SingletonContext) {
 	ctx.Build(pctx, BuildParams{
 		Rule:   soongDocs,
 		Output: docsFile,
+		Inputs: deps,
 		Args: map[string]string{
 			"outDir": PathForOutput(ctx, "docs").String(),
 		},