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