Update splash screen to properly use the Androidx SplashScreen library

[cbeyls]

The Androidx SplashScreen library is added as a dependency to the project but isn't properly enabled in the current code. This pull request configures the splash screen properly.

• Remove SplashScreenActivity which is not needed and use MainActivity as main entry point to the application. MainActivity inherits from BaseActivity which already detects if no account is configured and redirects to LoginActivity if needed, just like SplashScreenActivity.
• Initialize the SplashScreen library in MainActivity.onCreate().
• Instead of letting the SplashScreen library set the final theme from the postSplashScreenTheme attribute in SplashTheme, let BaseActivity set it according to the user settings.
• When no account is available in MainActivity.onCreate(), keep the splash screen shown until LoginActivity appears.
• Disable the slide-in animation when launching LoginActivity when no account is available because the detection happens in onCreate() and an Activity that finishes itself in onCreate() will not be drawn, so the slide-in animation will not be visible either and only LoginActivity will appear.