PostIt.Android: drive the PKCE flow through Chrome Custom Tabs

The earlier commit removed the client_secret and wired
MainActivity.OnNewIntent to AndroidOidcCallbackSink, but
IdentityModel.OidcClient.LoginAsync still had no IBrowser to drive
the user-agent half of the flow. Without it, the desktop / browser
projects continue to fail at login with 'No browser is available'.

Android now plugs in Chrome Custom Tabs:

  * PostIt.Android/Services/AndroidSystemBrowser.cs implements
    IBrowser.InvokeAsync using CustomTabsIntent.LaunchUrl and waits
    for MainActivity.AndroidOidcCallbackSink to deliver the deep-link
    Intent (android://postit-signin?code=...&state=...).
  * PostIt/Services/Platform.cs is a tiny static indirection the
    shared library uses to ask the running platform for an
    IBrowser and the appropriate default RedirectUri, without
    referencing any UI framework from the shared assembly.
  * LoginPageViewModel reads Platform.DefaultRedirectUri and
    Platform.CreateBrowser().Invoke() before calling LoginAsync.
  * PostIt.Android/PlatformBootstrap.cs wires the Android side at
    startup, and MainActivity.OnCreate calls EnsureInitialized().
  * Xamarin.AndroidX.Browser 1.8.0 added to the central package
    versions so CustomTabsIntent resolves.

src/PostIt/PostIt.Android/PostIt.Android.csproj

@@ -21,6 +21,7 @@
   <ItemGroup>
     <PackageReference Include="Avalonia.Android" />
     <PackageReference Include="Xamarin.AndroidX.Core.SplashScreen" />
+    <PackageReference Include="Xamarin.AndroidX.Browser" />
   </ItemGroup>
   <ItemGroup>
     <ProjectReference Include="..\PostIt\PostIt.csproj" />