Stop caching preferences instance (#20445)

It appears that reusing the shared preferences instance sometimes causes changes to not be properly committed to disk as the instance is not disposed in some cases before the app is torn down (eg: force close).

Caching isn't really necessary here anyways as the underlying android preference manager does its own layer of caching (so we are just paying the interop tax to get a 'new' instance here).

The simplest fix is to stop caching which is the real content of this change.  We don't cache in the regular preferences API where we use shared preferences and the issue does not seem to exist there.

src/Essentials/src/SecureStorage/SecureStorage.android.cs

@@ -15,9 +15,11 @@ Task<string> PlatformGetAsync(string key)
 			{
 				try
 				{
-					ISharedPreferences sharedPreferences = GetEncryptedSharedPreferences();
-					if (sharedPreferences != null)
-						return sharedPreferences.GetString(key, null);
+					var prefs = GetEncryptedSharedPreferences();
+					if (prefs != null)
+					{
+						return prefs.GetString(key, null);
+					}
 
 					// TODO: Use Logger here?
 					System.Diagnostics.Debug.WriteLine(
@@ -48,53 +50,52 @@ Task PlatformSetAsync(string key, string data)
 		{
 			return Task.Run(() =>
 			{
-				using ISharedPreferencesEditor editor = GetEncryptedSharedPreferences()?.Edit();
+				using var prefs = GetEncryptedSharedPreferences();
+				using var editor = prefs?.Edit();
 				if (data is null)
+				{
 					editor?.Remove(key);
+				}
 				else
+				{
 					editor?.PutString(key, data);
+				}
 
 				editor?.Apply();
 			});
 		}
 
 		bool PlatformRemove(string key)
 		{
-			using ISharedPreferencesEditor editor = GetEncryptedSharedPreferences()?.Edit();
+			using var prefs = GetEncryptedSharedPreferences();
+			using var editor = prefs?.Edit();
 			editor?.Remove(key)?.Apply();
 			return true;
 		}
 
 		void PlatformRemoveAll()
 		{
-			using var editor = PreferencesImplementation.GetSharedPreferences(Alias).Edit();
+			using var prefs = GetEncryptedSharedPreferences();
+			using var editor = prefs?.Edit();
 			editor?.Clear()?.Apply();
 		}
 
-		ISharedPreferences _prefs;
 		ISharedPreferences GetEncryptedSharedPreferences()
 		{
-			if (_prefs is not null)
-			{
-				return _prefs;
-			}
-
 			try
 			{
 				var context = Application.Context;
 
-				MasterKey prefsMainKey = new MasterKey.Builder(context, Alias)
+				var prefsMainKey = new MasterKey.Builder(context, Alias)
 					.SetKeyScheme(MasterKey.KeyScheme.Aes256Gcm)
 					.Build();
 
-				var sharedPreferences = EncryptedSharedPreferences.Create(
+				return EncryptedSharedPreferences.Create(
 					context,
 					Alias,
 					prefsMainKey,
 					EncryptedSharedPreferences.PrefKeyEncryptionScheme.Aes256Siv,
 					EncryptedSharedPreferences.PrefValueEncryptionScheme.Aes256Gcm);
-
-				return _prefs = sharedPreferences;
 			}
 			catch (InvalidProtocolBufferException)
 			{