Move environment staleness check to soong_ui.

Also delete the now-unnecessary soong_env binary.

Test: Manually. Also checked what happens when a used environment
variable changes.

Change-Id: Ib393e7f444e94819198c6cce4bcd8719d9fd9441
diff --git a/android/env.go b/android/env.go
index c2a09aa..a8c7777 100644
--- a/android/env.go
+++ b/android/env.go
@@ -21,7 +21,7 @@
 	"strings"
 	"syscall"
 
-	"android/soong/env"
+	"android/soong/shared"
 )
 
 // This file supports dependencies on environment variables.  During build manifest generation,
@@ -113,7 +113,7 @@
 		return
 	}
 
-	data, err := env.EnvFileContents(envDeps)
+	data, err := shared.EnvFileContents(envDeps)
 	if err != nil {
 		ctx.Errorf(err.Error())
 	}