AAPT2: Allow undefined resources (placeholders)

A resource defined like so:

<item type="drawable" name="foo" />

should be assigned the value @null.

The only exception is for <string> resources, which are given the
empty string value (since <string></string> is ambiguous). The decision
to use "" is based off the fact that old AAPT used to assign "" to all
undefined resources, even non-string ones.

Bug: 38425050
Test: make aapt2_tests
Change-Id: Ib3e0f6f83d16ddd8b279c9fd44a07a37867b85e9
diff --git a/tools/aapt2/ResourceParser.h b/tools/aapt2/ResourceParser.h
index 8258019..5631dc2 100644
--- a/tools/aapt2/ResourceParser.h
+++ b/tools/aapt2/ResourceParser.h
@@ -78,42 +78,32 @@
    * Item, then a
    * RawString is returned. Otherwise this returns false;
    */
-  std::unique_ptr<Item> ParseXml(xml::XmlPullParser* parser,
-                                 const uint32_t type_mask,
+  std::unique_ptr<Item> ParseXml(xml::XmlPullParser* parser, const uint32_t type_mask,
                                  const bool allow_raw_value);
 
   bool ParseResources(xml::XmlPullParser* parser);
   bool ParseResource(xml::XmlPullParser* parser, ParsedResource* out_resource);
 
-  bool ParseItem(xml::XmlPullParser* parser, ParsedResource* out_resource,
-                 uint32_t format);
+  bool ParseItem(xml::XmlPullParser* parser, ParsedResource* out_resource, uint32_t format);
   bool ParseString(xml::XmlPullParser* parser, ParsedResource* out_resource);
 
   bool ParsePublic(xml::XmlPullParser* parser, ParsedResource* out_resource);
-  bool ParsePublicGroup(xml::XmlPullParser* parser,
-                        ParsedResource* out_resource);
-  bool ParseSymbolImpl(xml::XmlPullParser* parser,
-                       ParsedResource* out_resource);
+  bool ParsePublicGroup(xml::XmlPullParser* parser, ParsedResource* out_resource);
+  bool ParseSymbolImpl(xml::XmlPullParser* parser, ParsedResource* out_resource);
   bool ParseSymbol(xml::XmlPullParser* parser, ParsedResource* out_resource);
-  bool ParseAddResource(xml::XmlPullParser* parser,
-                        ParsedResource* out_resource);
+  bool ParseAddResource(xml::XmlPullParser* parser, ParsedResource* out_resource);
   bool ParseAttr(xml::XmlPullParser* parser, ParsedResource* out_resource);
-  bool ParseAttrImpl(xml::XmlPullParser* parser, ParsedResource* out_resource,
-                     bool weak);
+  bool ParseAttrImpl(xml::XmlPullParser* parser, ParsedResource* out_resource, bool weak);
   Maybe<Attribute::Symbol> ParseEnumOrFlagItem(xml::XmlPullParser* parser,
                                                const android::StringPiece& tag);
   bool ParseStyle(const ResourceType type, xml::XmlPullParser* parser,
                   ParsedResource* out_resource);
   bool ParseStyleItem(xml::XmlPullParser* parser, Style* style);
-  bool ParseDeclareStyleable(xml::XmlPullParser* parser,
-                             ParsedResource* out_resource);
+  bool ParseDeclareStyleable(xml::XmlPullParser* parser, ParsedResource* out_resource);
   bool ParseArray(xml::XmlPullParser* parser, ParsedResource* out_resource);
-  bool ParseIntegerArray(xml::XmlPullParser* parser,
-                         ParsedResource* out_resource);
-  bool ParseStringArray(xml::XmlPullParser* parser,
-                        ParsedResource* out_resource);
-  bool ParseArrayImpl(xml::XmlPullParser* parser, ParsedResource* out_resource,
-                      uint32_t typeMask);
+  bool ParseIntegerArray(xml::XmlPullParser* parser, ParsedResource* out_resource);
+  bool ParseStringArray(xml::XmlPullParser* parser, ParsedResource* out_resource);
+  bool ParseArrayImpl(xml::XmlPullParser* parser, ParsedResource* out_resource, uint32_t typeMask);
   bool ParsePlural(xml::XmlPullParser* parser, ParsedResource* out_resource);
 
   IDiagnostics* diag_;