[Xamarin.Android.Bcl-Tests] Keep ActivationServices.CreateProxyForType (#936)

The results reporting acted weird and hid 2 failures, which were not
reported in the instrumentation result lines

	INSTRUMENTATION_RESULT: failed=0
	INSTRUMENTATION_RESULT: inconclusive=0
	INSTRUMENTATION_RESULT: passed=20134
	INSTRUMENTATION_RESULT: run=20466
	INSTRUMENTATION_RESULT: nunit2-results-path=/data/data/Xamarin.Android.Bcl_Tests/files/.__override__/TestResults.xml
	INSTRUMENTATION_RESULT: skipped=322
	INSTRUMENTATION_CODE: -1

and were only reported in the
`TestResult-Xamarin.Android.Bcl_Tests-Release.xml` file.

From `adb logcat` output we can see the crash (same for both failures):

	--------- beginning of crash
	E/AndroidRuntime( 3826): FATAL EXCEPTION: main
	E/AndroidRuntime( 3826): Process: Xamarin.Android.Bcl_Tests, PID: 3826
	E/AndroidRuntime( 3826): android.runtime.JavaProxyThrowable: System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---> System.NotSupportedException: Linked away.
	E/AndroidRuntime( 3826):   at (wrapper managed-to-native) System.Object:__icall_wrapper_ves_icall_object_new_specific (intptr)
	E/AndroidRuntime( 3826):   at MonoTests.System.Runtime.Remoting.ContextTest..ctor () [0x00000] in <0484cb939a1a4a72be4938b3c08edcaa>:0
	E/AndroidRuntime( 3826):   at (wrapper managed-to-native) System.Reflection.MonoCMethod:InternalInvoke (System.Reflection.MonoCMethod,object,object[],System.Exception&)
	E/AndroidRuntime( 3826):   at System.Reflection.MonoCMethod.InternalInvoke (System.Object obj, System.Object[] parameters) [0x00002] in <0e1d684ae38a4822aaf6364f06390ad6>:0
	E/AndroidRuntime( 3826):    --- End of inner exception stack trace ---

That lead us to [`mono/mono/metadata/object.c`][object.c] source,
where `mono_error_set_not_supported (error, "Linked away.");` is
used 6 times.

[object.c]: https://github.com/mono/mono/blob/b263d37/mono/metadata/object.c#L5303

One of the cases was related to:

```c
im = mono_class_get_method_from_name (klass, "CreateProxyForType", 1);
```

That led us to the linked away
`System.Runtime.Remoting.Activation.ActivationServices.CreateProxyForType()`
method.

Update `Xamarin.Android.Bcl-Tests/Resources/LinkerDescription.xml` to
preserve `ActivationServices.CreateProxyForType()`, allowing these
tests to succeed in a `LinkSDKOnly` environment.

tests/Xamarin.Android.Bcl-Tests/Resources/LinkerDescription.xml

@@ -43,6 +43,9 @@
     <type fullname="System.Runtime.InteropServices.DllImportAttribute">
       <method name=".ctor"/>
     </type>
+    <type fullname="System.Runtime.Remoting.Activation.ActivationServices">
+      <method name="CreateProxyForType"/>
+    </type>
     <type fullname="System.ConsoleKey"/>
     <type fullname="System.ConsoleKeyInfo"/>
   </assembly>