Implement cleanups requested in aosp/1818245:

- Remove useValidations: whenever it was used, it was turned on, so
there was no point in keeping that knob
- Use the new soong_build invocation name constants

Test: Presubmits.
Change-Id: Ia3e8928d30d3de1e0adaa741111771304e56b139

ui/build/config.go

@@ -784,6 +784,10 @@ func (c *configImpl) NamedGlobFile(name string) string {
 	return shared.JoinPath(c.SoongOutDir(), ".bootstrap/build-globs."+name+".ninja")
 }
 
+func (c *configImpl) UsedEnvFile(tag string) string {
+	return shared.JoinPath(c.SoongOutDir(), usedEnvFile+"."+tag)
+}
+
 func (c *configImpl) MainNinjaFile() string {
 	return shared.JoinPath(c.SoongOutDir(), "build.ninja")
 }