Use a unified ninja builddir

Instead of calling SetNinjaBuildDir, pass it to bootstrap.bash, so that
the bootstrap package can set it consistently during bootstrapping and
normal execution.

Bug: 63720725
Test: m -j nothing
Test: mkdir o; ../bootstrap.bash; ./soong
Change-Id: Ica88d2d5f1461b5be49bfe6316c6ec4ef4d89d49

android/androidmk.go

@@ -61,8 +61,6 @@ func (c *androidMkSingleton) GenerateBuildActions(ctx blueprint.SingletonContext
 		return
 	}
 
-	ctx.SetNinjaBuildDir(pctx, filepath.Join(config.buildDir, ".."))
-
 	var androidMkModulesList []Module
 
 	ctx.VisitAllModules(func(module blueprint.Module) {

ui/build/soong.go

@@ -24,6 +24,7 @@ func runSoongBootstrap(ctx Context, config Config) {
 
 	cmd := Command(ctx, config, "soong bootstrap", "./bootstrap.bash")
 	cmd.Environment.Set("BUILDDIR", config.SoongOutDir())
+	cmd.Environment.Set("NINJA_BUILDDIR", config.OutDir())
 	cmd.Sandbox = soongSandbox
 	cmd.Stdout = ctx.Stdout()
 	cmd.Stderr = ctx.Stderr()