Bump to xamarin-android-api-compatibility/d15-6/9d71e200 (#1207)

Context: #1089
Context: #1078
Context: https://jenkins.mono-project.com/view/Xamarin.Android/job/xamarin-android/823/

Commit 8ff139f [broke][0] the [build][1], in a minor way:

[0]: https://jenkins.mono-project.com/view/Xamarin.Android/job/xamarin-android/824/console
[1]: https://jenkins.mono-project.com/view/Xamarin.Android/job/xamarin-android/824/

	# reading extras from: inter-api-extra-v2.3-v4.0.3.txt
	mono --debug /Users/builder/jenkins/workspace/xamarin-android/xamarin-android/bin/BuildDebug/mono-api-html.exe "inter-apis/v2.3/Mono.Android.xml" "inter-apis/v4.0.3/Mono.Android.xml" --ignore-changes-parameter-names --ignore-changes-virtual --ignore-changes-property-setters --ignore-nonbreaking
	<h1>### API BREAK BETWEEN v2.3 and v4.0.3</h1>
	...
	<h2>Namespace Android.Text</h2>
	<!-- start type ClipboardManager --> <div>
	<h3>Type Changed: Android.Text.ClipboardManager</h3>
	<p>Modified properties:</p>
	<pre>
	<div data-is-breaking>	public <span class='added added-breaking-inline'>abstract</span> bool HasText { get; }
	</div><div data-is-breaking>	public <span class='added added-breaking-inline'>abstract</span> Java.Lang.ICharSequence TextFormatted { get; set; }
	</div></pre>
	...

These are "acceptable" API breakage for now, previously not reported
[because the test was broken][6dfb]. The test was fixed on
xamarin-android-api-compatibility/master, but we haven't been able to
merge to that commit yet (PR #1078).

[6dfb]: xamarin/xamarin-android-api-compatibility@6dfba92

Unfortunately, 8ff139f included a bump to
xamarin-android-api-compatibility/master, thus introducing the
"UNSTABLE" breakage that PR #1078 encountered.

For now -- until we can properly fix things -- bump to
xamarin-android-api-compatibility/d15-6, which *lacks* the test fixes
on master, and thus will allow the API compatibility checks to pass.

.gitmodules

@@ -36,7 +36,7 @@
 [submodule "external/xamarin-android-api-compatibility"]
 	path = external/xamarin-android-api-compatibility
 	url = https://github.com/xamarin/xamarin-android-api-compatibility.git
-	branch = master
+	branch = d15-6
 [submodule "external/xamarin-android-tools"]
 	path = external/xamarin-android-tools
 	url = https://github.com/xamarin/xamarin-android-tools