AAPT2: Add --package-id flag for feature-split suppport

Bug: 35928935
Change-Id: Ia8496505e61cfcfdb8f9f62366d2f36e453db111
Test: make aapt2_tests
diff --git a/tools/aapt2/diff/Diff.cpp b/tools/aapt2/diff/Diff.cpp
index c877468..dacf8d9 100644
--- a/tools/aapt2/diff/Diff.cpp
+++ b/tools/aapt2/diff/Diff.cpp
@@ -331,7 +331,7 @@
 
   void Visit(Reference* ref) override {
     if (ref->name && ref->id) {
-      if (ref->id.value().package_id() == 0x7f) {
+      if (ref->id.value().package_id() == kAppPackageId) {
         ref->id = {};
       }
     }