feat: allow disabling splash screen for embedded Cordova

[erisu]

Motivation and Context

Provide a way for native apps that embed a Cordova WebView to disable Cordova's Splash Screen.

This is not intended for apps created with Cordova CLI.

Description

Added showInitialSplashScreen method to return true by default.

Native apps that embed a Cordova WebView can override this method inside the Activity that extends CordovaActivity.

E.g.

override fun showInitialSplashScreen(): Boolean {
    return false
}

Testing

• Built native app
• Override method showInitialSplashScreen

Checklist

• I've run the tests to see all new and existing tests pass
• I added automated test coverage as appropriate for this change
• Commit is prefixed with (platform) if this change only applies to one platform (e.g. (android))
• If this Pull Request resolves an issue, I linked to the issue in the text above (and used the correct keyword to close issues using keywords)
• I've updated the documentation if necessary

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.24%. Comparing base (0190d2e) to head (ec074f5).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1824   +/-   ##
=======================================
  Coverage   72.24%   72.24%           
=======================================
  Files          23       23           
  Lines        1841     1841           
=======================================
  Hits         1330     1330           
  Misses        511      511           

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[dpogue]

This seems okay, since we don't support splashscreen.show() on Android