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/cc/gen.go b/cc/gen.go
index 7516b28..e5c4194 100644
--- a/cc/gen.go
+++ b/cc/gen.go
@@ -99,7 +99,7 @@
 
 	cmd.Text("BISON_PKGDATADIR=prebuilts/build-tools/common/bison").
 		FlagWithInput("M4=", ctx.Config().PrebuiltBuildTool(ctx, "m4")).
-		Tool(ctx.Config().PrebuiltBuildTool(ctx, "bison")).
+		PrebuiltBuildTool(ctx, "bison").
 		Flag("-d").
 		Flags(flags).
 		FlagWithOutput("--defines=", headerFile).
@@ -121,7 +121,7 @@
 	headerBp := outDir.Join(ctx, aidlPackage, "Bp"+shortName+".h")
 
 	cmd := rule.Command()
-	cmd.Tool(ctx.Config().HostToolPath(ctx, "aidl-cpp")).
+	cmd.BuiltTool(ctx, "aidl-cpp").
 		FlagWithDepFile("-d", depFile).
 		Flag("--ninja").
 		Flag(aidlFlags).