[enumification][Mono.Android] fix ChoiceMode enum generation.

[atsushieno]

After the last API XML generation sanitization, ChoiceMode was removed
because they were generated from AbsListView fields which DOES NOT EXIST
in API Level 10.

And getChoiceMode() and setChoiceMode are removed because they expect
that nonexistent enum.

Fortunately they are in ListView in API Level 10, so they can be generated
from there.

[atsushieno]

This should fix #1170 (comment)

[atsushieno]

We will regret this fix because this will be broken once we remove API Level 10.

[jonpryor]

macOS+xbuild fails because of an ABI break:

ABI BREAK IN: Mono.Android.dll
<!-- start namespace Android.Widget --> <div> 
<h2>Namespace Android.Widget</h2>
<!-- start type ListView --> <div>
<h3>Type Changed: Android.Widget.ListView</h3>
<p>Modified fields:</p>
<pre>
<div data-is-breaking>	public const <span class='removed removed-inline removed-breaking-inline'>int</span> <span class='added '>ChoiceMode</span> ChoiceModeMultiple = 2;
</div><div data-is-breaking>	public const <span class='removed removed-inline removed-breaking-inline'>int</span> <span class='added '>ChoiceMode</span> ChoiceModeNone = 0;
</div><div data-is-breaking>	public const <span class='removed removed-inline removed-breaking-inline'>int</span> <span class='added '>ChoiceMode</span> ChoiceModeSingle = 1;
</div></pre>

</div> <!-- end type ListView -->
</div> <!-- end namespace Android.Widget -->

[atsushieno]

They are false ABI breakage report. There is no run-time breakage unless you reference them by reflection.

[jonpryor]

xamarin/xamarin-android-api-compatibility#12 has been merged and cherry-picked to the d15-6 branch.

Please update this PR to include a bump to xamarin-android-api-compatibility/d15-6.