Add RuleBuilder helper functions for built and prebuilt tools

Replace the common pattern of:
cmd.Tool(ctx.Config().HostToolPath(ctx, "tool"))
with:
cmd.BuiltTool("tool")

And similarly for PrebuiltBuildTool.

Test: m checkbuild
Change-Id: I7d63188505362c7df6a3b3e7330b4a2cca5a2409
diff --git a/android/proto.go b/android/proto.go
index 5247c68..c8ade45 100644
--- a/android/proto.go
+++ b/android/proto.go
@@ -135,7 +135,7 @@
 	}
 
 	rule.Command().
-		Tool(ctx.Config().HostToolPath(ctx, "aprotoc")).
+		BuiltTool(ctx, "aprotoc").
 		FlagWithArg(flags.OutTypeFlag+"=", strings.Join(flags.OutParams, ",")+":"+outDir.String()).
 		FlagWithDepFile("--dependency_out=", depFile).
 		FlagWithArg("-I ", protoBase).
@@ -145,5 +145,5 @@
 		ImplicitOutputs(outputs)
 
 	rule.Command().
-		Tool(ctx.Config().HostToolPath(ctx, "dep_fixer")).Flag(depFile.String())
+		BuiltTool(ctx, "dep_fixer").Flag(depFile.String())
 }