meson: Key whether to build batch decoder on expat

Instead of on Android. Which allows an end user to turn off expat
without breaking or disabling Intel support. I've additionally
refactored to separate expat and xmlconfig a bit more in the root
meson.build

This does make expat a hard dependency for building Intel tools, despite
the fact that only aubinator actually requires it. This simplifies the
build for the common case, and in the event that someone wants to build
the Intel tools and doesn't have libexpat, they can fall back to the
meson wrap for expat instead.

fixes: 75276deebc
closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/8791

Reviewed-by: Mark Janes <markjanes@swizzler.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/23605>

meson_options.txt

@@ -58,6 +58,14 @@ option(
                 'separated list. Default: dri-drivers-path.'
 )
 
+option(
+  'expat',
+  type : 'feature',
+  value : 'auto',
+  description : 'Controls the use of expat. ' +
+                'Cannot be disabled if xmlconfig is enabled.'
+)
+
 option(
   'gallium-drivers',
   type : 'array',
@@ -649,6 +657,7 @@ option(
                 'the default driconf file is hardcoded into Mesa. ' +
                 'Requires expat.'
 )
+
 option (
   'intel-xe-kmd',
   type : 'feature',