Add <macro> tag to aapt2

AAPT2 Macros are compile-time resources definitions that are expanded
when referenced during the link phase.

A macro must be defined in the res/values.xml directory. A macro
definition for a macro named "foo" looks like the following:
 <macro name="foo">contents</macro>

When "@macro/foo" is used in the res/values directory or in a compiled
XML file, the contents of the macro replace the macro reference and
then the substituted contents are compiled and linked. If the macro
contents reference xml namespaces from its original definition, the
namespaces of the original macro definition will be used to determine
which package is being referenced.

Macros can be used anywhere resources can be referenced using the
@package:type/entry syntax.

Macros are not included in the final resource table or the R.java since
they are not actual resources.

Bug: 175616308
Test: aapt2_tests
Change-Id: I48b29ab6564357b32b4b4e32bff7ef06036382bc