Consistently use Bool instead of proptools.Bool

Use Bool instead of proptools.Bool and String instead of proptools.String.

Test: m checkbuild
Change-Id: I32d84add9f27128c7a65413e9612fd920613584f
diff --git a/android/arch.go b/android/arch.go
index 3990d7b..9f03f1b 100644
--- a/android/arch.go
+++ b/android/arch.go
@@ -854,17 +854,17 @@
 		addTarget(BuildOs, *variables.HostSecondaryArch, nil, nil, nil)
 	}
 
-	if config.Host_bionic != nil && *config.Host_bionic {
+	if Bool(config.Host_bionic) {
 		addTarget(LinuxBionic, "x86_64", nil, nil, nil)
 	}
 
-	if variables.CrossHost != nil && *variables.CrossHost != "" {
+	if String(variables.CrossHost) != "" {
 		crossHostOs := osByName(*variables.CrossHost)
 		if crossHostOs == NoOsType {
 			return nil, fmt.Errorf("Unknown cross host OS %q", *variables.CrossHost)
 		}
 
-		if variables.CrossHostArch == nil || *variables.CrossHostArch == "" {
+		if String(variables.CrossHostArch) == "" {
 			return nil, fmt.Errorf("No cross-host primary architecture set")
 		}