Remove terminal.Writer

terminal.Writer is now just a wrapper around stdio.Stdout() without
any useful functionality.  Replace it with stdio.Stdout() as an
io.Writer.

Test: ui/terminal/status_test.go
Change-Id: I5bc5476afdca950b505642f0135a3af9d37fbe24
diff --git a/ui/terminal/Android.bp b/ui/terminal/Android.bp
index 683e3e3..b533b0d 100644
--- a/ui/terminal/Android.bp
+++ b/ui/terminal/Android.bp
@@ -21,7 +21,7 @@
         "format.go",
         "smart_status.go",
         "status.go",
-        "writer.go",
+        "stdio.go",
         "util.go",
     ],
     testSrcs: [