EC-149 Fix crash when trying to Focus on Lock page

[fedemkr]

Type of change

• Bug fix
• New feature development
• Tech debt (refactoring, code cleanup, dependency upgrades, etc)
• Build/deploy pipeline (DevOps)
• Other

Objective

Fix crash produced when focussing the pin/master password entry on the lock page after choosing the fallback method on a failed biometric scan.
I couldn't reproduced this but there are a lot of crashes reported on AppCenter so just fixed it where it's appearing to crash.
Also improved the code so that the ViewModel have fewer direct accesses to the View/Page.

Code changes

• LockPage.xaml.cs: Improved the code making the SecretEntry centralizing the logic with the PinLock to get the correct entry. Also perform the logic of Focusing on the Page invoking it on the main thread to avoid the crash.
• LockPageViewModel.cs: Moved the focus action to the page, given that the VM should not do this directly on MVVM. Also added WeakEventManager so we don't have to worry about memory leaks.

Testing requirements

I couldn't reproduce the issue but given the crash report it would be like:

1. Enter you account
2. Configure biometrics
3. Configure pin
4. Lock the account
5. Try to unlock with fail biometric (i.e. don't use the correct one)
6. Try again until the fallback appears (Master Password / Pin)
7. Choose the fallback
8. Then it should crash

Also it could be like that but with the automatic prompting of biometrics when minimizing the app and starting it again.

Btw, if no biometrics is setup the pin/master password entry should be automatically focused (i.e. should work as before) and if biometrics are configured and the fallback is selected the same should happen, i.e. it should automatically focus the pin/master password.

Before you submit

• I have added unit tests where it makes sense to do so (encouraged but not required)
• This change requires a documentation update (notify the documentation team)
• This change has particular deployment requirements (notify the DevOps team)