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

ui/terminal/status.go

@@ -15,6 +15,8 @@
 package terminal
 
 import (
+	"io"
+
 	"android/soong/ui/status"
 )
 
@@ -24,10 +26,10 @@ import (
 //
 // statusFormat takes nearly all the same options as NINJA_STATUS.
 // %c is currently unsupported.
-func NewStatusOutput(w Writer, statusFormat string, quietBuild bool) status.StatusOutput {
+func NewStatusOutput(w io.Writer, statusFormat string, quietBuild bool) status.StatusOutput {
 	formatter := newFormatter(statusFormat, quietBuild)
 
-	if w.isSmartTerminal() {
+	if isSmartTerminal(w) {
 		return NewSmartStatusOutput(w, formatter)
 	} else {
 		return NewDumbStatusOutput(w, formatter)