AAPT2: Allow to inline XML into custom attribute

Previously, doing something like

<parent xmlns:app="http://schemas.android.com/apk/res-auto"
        xmlns:aapt="http://schemas.android.com/aapt">
    <aapt:attr name="app:foo">
        <child />
    </aapt:attr>
</parent>

would result in something like:

<parent xmlns:app="http://schemas.android.com/apk/res-auto"
        xmlns:aapt="http://schemas.android.com/aapt"
        foo="@generated_name" />

while it should result in:

<parent xmlns:app="http://schemas.android.com/apk/res-auto"
        xmlns:aapt="http://schemas.android.com/aapt"
        app:foo="@generated_name" />

Bug: 36809755
Test: make aapt2_tests
Change-Id: I72ea4b402e196ca05b53b788e4768a265190a0dc
diff --git a/tools/aapt2/xml/XmlDom_test.cpp b/tools/aapt2/xml/XmlDom_test.cpp
index 486b53a..ca46d53 100644
--- a/tools/aapt2/xml/XmlDom_test.cpp
+++ b/tools/aapt2/xml/XmlDom_test.cpp
@@ -154,6 +154,12 @@
       EXPECT_THAT(TransformPackageAlias("two"), Eq(make_value(ExtractedPackage{"com.two", false})));
       EXPECT_THAT(TransformPackageAlias("three"),
                   Eq(make_value(ExtractedPackage{"com.three", false})));
+    } else if (el->name == "View4") {
+      EXPECT_THAT(TransformPackageAlias("one"), Eq(make_value(ExtractedPackage{"com.one", false})));
+      EXPECT_THAT(TransformPackageAlias("two"), Eq(make_value(ExtractedPackage{"com.two", false})));
+      EXPECT_THAT(TransformPackageAlias("three"),
+                  Eq(make_value(ExtractedPackage{"com.three", false})));
+      EXPECT_THAT(TransformPackageAlias("four"), Eq(make_value(ExtractedPackage{"", true})));
     }
   }
 };
@@ -162,7 +168,9 @@
   std::unique_ptr<XmlResource> doc = test::BuildXmlDom(R"(
       <View1 xmlns:one="http://schemas.android.com/apk/res/com.one">
         <View2 xmlns:two="http://schemas.android.com/apk/res/com.two">
-          <View3 xmlns:three="http://schemas.android.com/apk/res/com.three" />
+          <View3 xmlns:three="http://schemas.android.com/apk/res/com.three">
+            <View4 xmlns:four="http://schemas.android.com/apk/res-auto" />
+          </View3>
         </View2>
       </View1>)");