AAPT2: Few tweaks to get shared-libraries working

Test: manual (building shared support library demo)
Change-Id: I4730645aa92ba1893baf67ffe35fbd4aac0f8e46
diff --git a/tools/aapt2/process/SymbolTable.cpp b/tools/aapt2/process/SymbolTable.cpp
index 5d75e76..bcafbca 100644
--- a/tools/aapt2/process/SymbolTable.cpp
+++ b/tools/aapt2/process/SymbolTable.cpp
@@ -293,6 +293,11 @@
 
 std::unique_ptr<SymbolTable::Symbol> AssetManagerSymbolSource::FindById(
     ResourceId id) {
+  if (!id.is_valid()) {
+    // Exit early and avoid the error logs from AssetManager.
+    return {};
+  }
+
   const android::ResTable& table = assets_.getResources(false);
   Maybe<ResourceName> maybe_name = GetResourceName(table, id);
   if (!maybe_name) {