[Xamarin.Android.Build.Tasks] Add XA0117 warning (#2250)

Add a new XA0117 warning which will inform users if they are using a
`$(TargetFrameworkVersion)` value which will be deprecated "soon".

In our next version our minimum supported `$(TargetFrameworkVersion)`
will be v4.4 (API-19), and support for v2.3 (API-10) will be removed.

Author: dellis1972

Documentation/guides/messages/README.md

@@ -41,6 +41,8 @@
 + [XA0113](xa0113.md): Google Play requires that new applications must use a `$(TargetFrameworkVersion)` of v8.0 (API level 26) or above.
 + [XA0114](xa0114.md): Google Play requires that application updates must use a `$(TargetFrameworkVersion)` of v8.0 (API level 26) or above.
 + [XA0115](xa0115.md): Invalid value 'armeabi' in $(AndroidSupportedAbis). This ABI is no longer supported. Please update your project properties.
++ [XA0116](xa0116.md): Unable to find `EmbeddedResource` of name `{ResourceName}`.
++ [XA0117](xa0117.md): The TargetFrameworkVersion {TargetFrameworkVersion} is deprecated. Please update it to be v4.4 or higher.
 
 ### XA1xxx Project Related
 

Documentation/guides/messages/xa0117.md

@@ -0,0 +1,25 @@
+# Compiler Warning XA0117
+
+As the Android platform evolves certain versions will be deprecated and
+removed. How we decide if a platform is deprecated largely depends on whether
+the [Android NDK][ndk] continues to support the platform in question.
+It also depends on whether the
+[Android Support Libraries support that platform][support].
+
+If both the Android NDK and the Android Support Libraries have removed support
+for a particular `apiLevel`, then it is highly likely that support for the
+API level will be deprecated and removed from Xamarin.Android.
+
+If you are seeing this warning it is because your `$(TargetFrameworkVersion)`
+is set to a value which is likely to be deprecated in the near future.
+To remove this warning update your `$(TargetFrameworkVersion)` to use
+the latest stable platform. 
+
+Your `android:minSdk` value within `AndroidManifest.xml` can still mention
+older versions of the android platform.
+
+
+[ndk]: https://developer.android.com/ndk/downloads/revision_history
+[support]: https://developer.android.com/distribute/best-practices/develop/target-sdk
+
+

src/Xamarin.Android.Build.Tasks/Tasks/ResolveAndroidTooling.cs

@@ -190,6 +190,8 @@ public override bool Execute ()
 					Log.LogCodedWarning ("XA0113", $"Google Play requires that new applications must use a TargetFrameworkVersion of v8.0 (API level 26) or above. You are currently targeting {TargetFrameworkVersion} (API level {AndroidApiLevel}).");
 				if (apiLevel < 26)
 					Log.LogCodedWarning ("XA0114", $"Google Play requires that application updates must use a TargetFrameworkVersion of v8.0 (API level 26) or above. You are currently targeting {TargetFrameworkVersion} (API level {AndroidApiLevel}).");
+				if (apiLevel < 19)
+					Log.LogCodedWarning ("XA0117", $"The TargetFrameworkVersion {TargetFrameworkVersion} is deprecated. Please update it to be v4.4 or higher.");
 			}
 
 			SequencePointsMode mode;