AAPT2: Strip dedicated tools namespace from XML

Android has a dedicated XML namespace dedicated for tools that should not be
included in the final APK. AAPT strips this out, but the feature was missing
from AAPT2. See: http://tools.android.com/tech-docs/tools-attributes

Bug: 29115919
Change-Id: I8f4fc79e6c8592a313a691134e44d16fd91f36ed
diff --git a/tools/aapt2/xml/XmlUtil.h b/tools/aapt2/xml/XmlUtil.h
index f0d59b7..b75d8ac 100644
--- a/tools/aapt2/xml/XmlUtil.h
+++ b/tools/aapt2/xml/XmlUtil.h
@@ -29,6 +29,7 @@
 constexpr const char* kSchemaPublicPrefix = "http://schemas.android.com/apk/res/";
 constexpr const char* kSchemaPrivatePrefix = "http://schemas.android.com/apk/prv/res/";
 constexpr const char* kSchemaAndroid = "http://schemas.android.com/apk/res/android";
+constexpr const char* kSchemaTools = "http://schemas.android.com/tools";
 
 /**
  * Result of extracting a package name from a namespace URI declaration.