Change the default Cordova splash screen to be more generic

Cordova's current default splash screen is heavily Cordova-branded. If this splash screen accidentally makes its way into production (e.g. for some corner case that developers had not encountered during testing), users end up seeing a totally alien logo/brand that has nothing to do with the app they're using, and they can easily get confused and disoriented.

This effect would be significantly reduced by changing the default splash screen that is shown when no alternative one can be found in configuration to something much more generic, e.g. a black background with just a "loading" spinner, or the text "Loading...". (Even hardcoding this text in English would be much more within users' ideas of "normal" behavior compared to what they see now, even if they don't have their device set to English, and therefore greatly preferable to the current behavior.)

Note, this was inspired by surprises like https://issues.apache.org/jira/browse/CB-12947 and https://issues.apache.org/jira/browse/CB-12952 – due to such things, it's currently more likely than it should be for developers to let the default Cordova splash screen sneak into production. Therefore it is particularly desirable to minimize the impact to users when this happens.

Thanks for your consideration.