Progress towards efficient XML serialization.

We've identified that XML writing and reading uses roughly 1.5% of
all system_server CPU, and can generate many temporary objects.

To set the stage for some long-term improvements, this change
introduces new TypedXmlSerializer and TypedXmlPullParser interfaces
which offer efficient access to primitive attributes and text.

This change also updates XmlUtils to redirect primitive operations
through these new interfaces.

Bug: 171832118
Test: atest FrameworksCoreTests:android.util.XmlTest
Change-Id: I5ba3ad87cf79ca10705a96b98855164a27fee021