Add support for CMakefile generation

Test: Manually generated CMakeLists.txt for gui/ui/aapt2.

Change-Id: I7dedc300c1e50b8e39bc58091b650c0bbe2c62da
diff --git a/android/register.go b/android/register.go
index 0ad9d30..d6b290d 100644
--- a/android/register.go
+++ b/android/register.go
@@ -77,6 +77,7 @@
 			handle.Parallel()
 		}
 	}
+	ctx.RegisterSingletonType("env", EnvSingleton)
 
 	return ctx
 }