Fix ChoiceMode field types to match expected ones. (#12)

Context: dotnet/android#1170
Context: dotnet/android#1183
Context: dotnet/android@a301764

Xamarin.Android 8.1 (d15-5) provides an `Android.Widget.ChoiceMode`
enum type in API-10+. `ChoiceMode` contains the
`android.widget.ListView.CHOICE_MODE_*` constants from API-10, which
were moved to `android.widget.AbsListView` in API-15.

Xamarin.Android 8.2 (d15-6) inadvertently *removed* the
`Android.Widget.ChoiceMode` enum, because
[xamarin-android/a301764a][xa-a301764a] altered the mapping of the
`CHOICE_MODE` constants so that they were only bound into the
`Android.Widget.ChoiceMode` enum for API-15+.

[xa-a301764a]: dotnet/android@a301764

[xamarin-android PR #1183][xa-1183] fixes `ChoiceMode` generation so
that it applies to API-10+, and in the process alters the API tracked
within xamarin-android-api-compatibility.

[xa-1183]: dotnet/android#1183

Add the `ChoiceMode` API changes so that [PR #1183][xa-1183] passes
the API compatibility tests.

reference/Mono.Android.xml

@@ -776331,7 +776331,7 @@
               <interface name="System.IDisposable" />
             </interfaces>
             <fields>
-              <field name="ChoiceModeMultiple" attrib="32854" fieldtype="System.Int32" value="2">
+              <field name="ChoiceModeMultiple" attrib="32854" fieldtype="Android.Widget.ChoiceMode" value="2">
                 <attributes>
                   <attribute name="Android.Runtime.RegisterAttribute">
                     <properties>
@@ -776340,7 +776340,7 @@
                   </attribute>
                 </attributes>
               </field>
-              <field name="ChoiceModeNone" attrib="32854" fieldtype="System.Int32" value="0">
+              <field name="ChoiceModeNone" attrib="32854" fieldtype="Android.Widget.ChoiceMode" value="0">
                 <attributes>
                   <attribute name="Android.Runtime.RegisterAttribute">
                     <properties>
@@ -776349,7 +776349,7 @@
                   </attribute>
                 </attributes>
               </field>
-              <field name="ChoiceModeSingle" attrib="32854" fieldtype="System.Int32" value="1">
+              <field name="ChoiceModeSingle" attrib="32854" fieldtype="Android.Widget.ChoiceMode" value="1">
                 <attributes>
                   <attribute name="Android.Runtime.RegisterAttribute">
                     <properties>
@@ -1107278,4 +1107278,4 @@
       </namespace>
     </namespaces>
   </assembly>
-</assemblies>
+</assemblies>