Add install_in_root to prebuilt_root module

This is to support installing to real root "/", not a root of partition
(e.g. "/system/").

Bug: 351258461
Test: build
Change-Id: I56993aceddfd988d2d10040f50f838b89c9bdc67

android/neverallow.go

@@ -237,6 +237,7 @@ func createInitFirstStageRules() []Rule {
 			Without("name", "init_first_stage").
 			Without("name", "init_first_stage.microdroid").
 			With("install_in_root", "true").
+			NotModuleType("prebuilt_root").
 			Because("install_in_root is only for init_first_stage."),
 	}
 }