[Xamarin.Android.Tools.AndroidSdk] Permit NDK r24

[jonpryor]

Update AndroidSdkBase.MaximumCompatibleNDKMajorVersion to 24, so
that NDK r24 is considered as a valid version.

[grendello]

It's fine for now, but I think this value should be settable by the users of the library (XA, IDEs etc)

[pjcollins]

Added #172 to track @grendello's comment.

[jonpryor]

@grendello: do we need a "max supported NDK" at all, here (in xamarin-android-tools)? It's not a public member, and thus nothing else can easily use it for anything, and it's not even used to verify constructor parameters, which will be specified "elsewhere" ( https://github.com/xamarin/xamarin-android/blob/944f88a8b6f1c30eaf43f56cfbebf0f8cd7a547f/src/Xamarin.Android.Build.Tasks/Utilities/MonoAndroidHelper.cs#L122 ).

Why does AndroidSdkInfo -- and not even all codepaths! just "we' re looking around randomly for an SDK & NDK" codepaths! -- need to check that the detected NDK is within a supported range?

[grendello]

@jonpryor it was added to prevent us from using NDKs we don't support (e.g. we didn't have support for the clang-only NDKs for a while) on the end user's machine. The idea was to yell loud if the user updated to latest NDK that we have no support for yet.