Add explicit rspfile argument to RuleBuilderCommand.FlagWithRspFileInputList

Using $out.rsp as the rsp file adds extra complexity around keeping
the $ unescaped.  Make callers to FlagWithRspFileInputList provide
an explicit path for the rsp file instead.

Bug: 182612695
Test: rule_builder_test.go
Change-Id: I3f531d80c1efa8a9d09aac0a63790c5b11a9f0c6
diff --git a/android/rule_builder_test.go b/android/rule_builder_test.go
index 06ea124..fbf624e 100644
--- a/android/rule_builder_test.go
+++ b/android/rule_builder_test.go
@@ -267,10 +267,10 @@
 	ctx := builderContext()
 	fmt.Println(NewRuleBuilder(pctx, ctx).Command().
 		Tool(PathForSource(ctx, "javac")).
-		FlagWithRspFileInputList("@", PathsForTesting("a.java", "b.java")).
-		NinjaEscapedString())
+		FlagWithRspFileInputList("@", PathForOutput(ctx, "foo.rsp"), PathsForTesting("a.java", "b.java")).
+		String())
 	// Output:
-	// javac @$out.rsp
+	// javac @out/foo.rsp
 }
 
 func ExampleRuleBuilderCommand_String() {