"Resolve" both human-readable and binary XML.

Recent refactorings have pivoted many XML users across the system
to call resolveSerializer() and resolvePullParser(), which gives us
an easy path to pivot between human-readable XML and our binary XML
wire format.

This change introduces a feature flag (still disabled by default)
for emitting binary XML by default when serializing.  It also adds
logic to automatically select the relevant parser by sniffing the
magic file header.

This is the key to our upgrade strategy: we'll gladly parse
human-readable XML from old devices, or binary XML from new devices.

Bug: 171832118
Test: atest FrameworksCoreTests:android.util.XmlTest
Test: atest FrameworksCoreTests:android.util.BinaryXmlTest
Change-Id: I14fdab9a6667819db38116a8ce126f5f2a5a04df