AAPT2: binary parser should be more lenient with files

Some APKs have file references that reference files that don't exist.

For now, just be lenient and ignore the files themselves.

Keep the actual reference until such a time as general resource
stripping is implemented.

Bug: 37749644
Test: manual
Change-Id: I773685acf41212405b71b064d64045fb4556dcfc
diff --git a/tools/aapt2/cmd/Optimize.cpp b/tools/aapt2/cmd/Optimize.cpp
index 78ed49b..194c0c8 100644
--- a/tools/aapt2/cmd/Optimize.cpp
+++ b/tools/aapt2/cmd/Optimize.cpp
@@ -213,10 +213,10 @@
 
             if (file_ref->file == nullptr) {
               ResourceNameRef name(pkg->name, type->type, entry->name);
-              context_->GetDiagnostics()->Error(DiagMessage(file_ref->GetSource())
+              context_->GetDiagnostics()->Warn(DiagMessage(file_ref->GetSource())
                                                 << "file for resource " << name << " with config '"
                                                 << config_value->config << "' not found");
-              return false;
+              continue;
             }
 
             const StringPiece entry_name = entry->name;