Custom binary XML wire protocol tools.

To support local development work, add new "xml2abx" and "abx2xml"
shell utilities which offer to convert between human-readable XML and
our new binary XML protocol.

Interally this utility uses Xml.copy() which is already covered by
existing unit tests that verify lossless conversion between the two
underlying protocols.

Bug: 171832118
Test: manual
Change-Id: I2f4c9850754cafb2bb31eeefa6bd611360963b03