Update soong to use pathtools.FileSystem

Update soong to follow changes in
https://github.com/google/blueprint/pull/141

Test: soong tests
Change-Id: I49a9b83cac7590dc75b26b31136b8707c188bc4a

android/package_ctx.go

@@ -18,6 +18,7 @@ import (
 	"fmt"
 
 	"github.com/google/blueprint"
+	"github.com/google/blueprint/pathtools"
 )
 
 // AndroidPackageContext is a wrapper for blueprint.PackageContext that adds
@@ -55,6 +56,10 @@ func (e *configErrorWrapper) AddNinjaFileDeps(deps ...string) {
 	e.pctx.AddNinjaFileDeps(deps...)
 }
 
+func (e *configErrorWrapper) Fs() pathtools.FileSystem {
+	return nil
+}
+
 // SourcePathVariable returns a Variable whose value is the source directory
 // appended with the supplied path. It may only be called during a Go package's
 // initialization - either from the init() function or as part of a