[generator] Availability attribute incorrectly added on existing types with new protocol conformance

[VincentDondain]

Steps to Reproduce

Add protocol AVQueuedSampleBufferRendering to AVSampleBufferDisplayLayer.

See generated code.

Expected Behavior

Since the type already exists and is working do not alter availability (that would be problematic for older OSes).

Actual Behavior

Availability is updated based on the fact that the protocol was added in a subsequent OS version. AVSampleBufferDisplayLayer was added in iOS 8 and AVQueuedSampleBufferRendering in iOS 11.

Environment

=== Visual Studio Community 2017 for Mac (Preview) ===

Version 7.4 Preview (7.4 build 720)
Installation UUID: 00d602ef-271b-479b-9054-0433b5797215
Runtime:
	Mono 5.8.0.99 (2017-10/e65bf00e22b) (64-bit)
	GTK+ 2.24.23 (Raleigh theme)

	Package version: 508000099

=== NuGet ===

Version: 4.3.1.4445

=== .NET Core ===

Runtime: /usr/local/share/dotnet/dotnet
Runtime Versions:
	2.0.0-preview2-25407-01
	1.1.2
	1.0.5
SDK: /usr/local/share/dotnet/sdk/2.0.0-preview2-006497/Sdks
SDK Versions:
	2.0.0-preview2-006497
	1.0.4
MSBuild SDKs: /Library/Frameworks/Mono.framework/Versions/5.8.0/lib/mono/msbuild/15.0/bin/Sdks

=== Xamarin.Profiler ===

Version: 1.5.6
Location: /Applications/Xamarin Profiler.app/Contents/MacOS/Xamarin Profiler

=== Apple Developer Tools ===

Xcode 9.2 (13772)
Build 9C40b

=== Xamarin.Mac ===

Version: 4.3.0.17 (Visual Studio Community)

=== Xamarin.iOS ===

Version: 11.9.0.17 (Visual Studio Community)
Hash: 6996ac0e
Branch: bug-59272
Build date: 2018-01-11 14:22:31-0500

=== Xamarin.Android ===

Version: 8.0.2.1 (Visual Studio Community)
Android SDK: /Users/vidondai/Library/Developer/Xamarin/android-sdk-macosx
	Supported Android versions:
		6.0 (API level 23)
		7.1 (API level 25)
		8.0 (API level 26)

SDK Tools Version: 25.2.5
SDK Platform Tools Version: 25.0.4
SDK Build Tools Version: 23.0.2

Java SDK: /usr
java version "1.8.0_101"
Java(TM) SE Runtime Environment (build 1.8.0_101-b13)
Java HotSpot(TM) 64-Bit Server VM (build 25.101-b13, mixed mode)

Android Designer EPL code available here:
https://github.com/xamarin/AndroidDesigner.EPL

=== Xamarin Inspector ===

Version: 1.3.2
Hash: 461f09a
Branch: 1.3-release
Build date: Tue, 03 Oct 2017 18:26:57 GMT
Client compatibility: 1

=== Build Information ===

Release ID: 704000720
Git revision: d5d46cb994643f64cc683737ecef76f581cf2b89
Build date: 2017-11-28 09:24:00-05
Xamarin addins: e7e3c0e88c36edf0498dba75a52e3c0654fe69ed
Build lane: monodevelop-lion-master

=== Operating System ===

Mac OS X 10.12.6
Darwin 16.7.0 Darwin Kernel Version 16.7.0
    Wed Oct  4 00:17:00 PDT 2017
    root:xnu-3789.71.6~1/RELEASE_X86_64 x86_64

=== Enabled user installed addins ===

NuGet Package Explorer 0.2
Internet of Things (IoT) development (Preview) 7.1