Frontend.XF.Android: fix NRE when scanning QR codes

This commit adds the missing Xamarin.Essential init call
that was causing the ZXing.Net to crash.

This regression was introduced by this commmit: [1]
which updated ZXing.Net.Xamarin from 0.10.x to 2.9.99,
which seems to introduce Xamarin.Essentials as dependency
and needs initializing on Android, as evidenced by: [2]

[1] https://gitlab.com/nblockchain/geewallet/-/commit/9e313fac173bc9d9eab19b90970cbea199dfb6b2
[2] Redth/ZXing.Net.Mobile@5ddfa0e

Stacktrace:
System.NullReferenceException: The current Activity can not be detected. Ensure that you have called Init in your Activity or Application class.
  at Xamarin.Essentials.Platform.GetCurrentActivity (System.Boolean throwOnNull) [0x00018] in D:\a\DotNetEssentials\DotNetEssentials\Xamarin.Essentials\Platform\Platform.android.cs:80
  at Xamarin.Essentials.Permissions+BasePlatformPermission.RequestAsync () [0x00208] in D:\a\DotNetEssentials\DotNetEssentials\Xamarin.Essentials\Permissions\Permissions.android.cs:118
  at ZXing.Net.Mobile.Forms.Android.ZXingScannerViewRenderer.OnElementChanged (Xamarin.Forms.Platform.Android.ElementChangedEventArgs`1[TElement] e) [0x0005d] in D:\a\ZXing.Net.Xamarin\ZXing.Net.Xamarin\ZXing.Net.Mobile.Forms\ZXingScannerViewRenderer.android.cs:59
  at GWallet.Backend.Infrastructure.Report (System.Exception ex, SharpRaven.Data.ErrorLevel _arg1) [0x00017] in C:\geewallet\src\GWallet.Backend\Infrastructure.fs:131
  at GWallet.Backend.Infrastructure.LogOrReportCrash (System.Exception ex) [0x00000] in C:\geewallet\src\GWallet.Backend\Infrastructure.fs:150
  at GWallet.Backend.Infrastructure.OnUnhandledException (System.Object _arg1, System.UnhandledExceptionEventArgs args) [0x0000c] in C:\geewallet\src\GWallet.Backend\Infrastructure.fs:162
  at [email protected] (System.Object delegateArg0, System.UnhandledExceptionEventArgs delegateArg1) <0x783f80cdf8 + 0x00043> in <94e6baede217a6cfa3c49f143dcc2e81>:0

See https://gitlab.com/nblockchain/geewallet/-/merge_requests/143

src/GWallet.Frontend.XF.Android/MainActivity.fs

@@ -32,6 +32,7 @@ type MainActivity() =
         base.OnCreate (bundle)
         Xamarin.Forms.Forms.Init (this, bundle)
 
+        Xamarin.Essentials.Platform.Init(this, bundle)
         ZXing.Net.Mobile.Forms.Android.Platform.Init()
 
         this.LoadApplication (new GWallet.Frontend.XF.App ())