AAPT2: Fix merging of styleables the right way

Styleables should only be merged when processing overlays.
This moves the styleable merging code out of ResourceTable
and into TableMerger.

Change-Id: I3aae05cf4dd875cd25ac2ac744b61194409b2fee
diff --git a/tools/aapt2/ResourceValues.h b/tools/aapt2/ResourceValues.h
index eb7559a..5e5d1f3 100644
--- a/tools/aapt2/ResourceValues.h
+++ b/tools/aapt2/ResourceValues.h
@@ -172,6 +172,7 @@
     Reference();
     explicit Reference(const ResourceNameRef& n, Type type = Type::kResource);
     explicit Reference(const ResourceId& i, Type type = Type::kResource);
+    explicit Reference(const ResourceNameRef& n, const ResourceId& i);
 
     bool equals(const Value* value) const override;
     bool flatten(android::Res_value* outValue) const override;