I was having the same issue with several Dependency Services

The simplest solution

In a lot of cases for Single Activity Applications

Xamarin.Forms.Forms.Context

Can be replaced with

Android.App.Application.Context

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The Background in more detail

Android.App.Application.Context returns the global Application Context of the current process tied to the lifecycle of the Application, as apposed to an Activity context.

A typical example of using the Application context is for starting an Activity e.g.

Android.App.Application.Context.StartActivity(myIntent);

The general rule of thumb is to use the current Activity Context, unless you need
to save a reference to a context from an object that lives beyond your
Activity. In which case use the Application context

Why did Forms.Context go obsolete?

Xmarin.Forms 2.5 introduced a new “Forms embedding” feature, which can embed Forms pages into Xamarin.iOS / Xamarin.Android apps. However, since Xamarin.Android apps can use multiple Activities, seemingly there was a danger of Xamarin.Android users calling Forms.Context and in turn getting a reference to the MainActivity, which has the potential cause problems.

The work around

Inside a Renderer you now get a reference to the view’s context which is passed into the constructor.

With any other class you are faced with the issue of how to get the Activity Context. In a single Activity application (in most cases) the Application.Context will work just fine.

However to get the current Activity Context in a Multiple Activity Application you will need to hold a reference to it. The easiest and most reliable way to do this is via a class that implements the Application.IActivityLifecycleCallbacks Interface.

The main idea is to keep a reference of the Context when an Activity
is created, started, or resumed.

[Application]
public partial class MainApplication : Application, Application.IActivityLifecycleCallbacks
{
   internal static Context ActivityContext { get; private set; }

   public MainApplication(IntPtr handle, JniHandleOwnership transfer) : base(handle, transfer) { }

   public override void OnCreate()
   {
      base.OnCreate();
      RegisterActivityLifecycleCallbacks(this);
   }    

   public override void OnTerminate()
   {
      base.OnTerminate();
      UnregisterActivityLifecycleCallbacks(this);
   }  

   public void OnActivityCreated(Activity activity, Bundle savedInstanceState)
   {
      ActivityContext = activity;
   }   

   public void OnActivityResumed(Activity activity)
   {
      ActivityContext = activity;
   }

   public void OnActivityStarted(Activity activity)
   {
      ActivityContext = activity;
   }

   public void OnActivityDestroyed(Activity activity) { }  
   public void OnActivityPaused(Activity activity) { }
   public void OnActivitySaveInstanceState(Activity activity, Bundle outState) { }
   public void OnActivityStopped(Activity activity) { }
}

With the above approach, single Activity Applications and multiple Activity Applications can now always have access to the Current/Local Activity Context. e.g instead of relying on the global context

Android.App.Application.Context 

// or previously

Xamarin.Forms.Forms.Context

Can now be replaced with

MainApplication.ActivityContext

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Example call in a Dependency Service

if (MainApplication.ActivityContext!= null)
{
    versionNumber = MainApplication.ActivityContext
                       .PackageManager
                       .GetPackageInfo(MainApplication.ActivityContext.PackageName, 0)
                       .VersionName;
}

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Additional Resources

Android.App.Application.IActivityLifecycleCallbacks

registerActivityLifecycleCallbacks