Well, I mean you wanted them you got themYou can try deleting the build-cache, which is located here: <user-home>/.android/build-cache/ (WARNING THIS COULD BREAK ANDROID STUDIO - CLOSE THE PROGRAM BEFORE DELETING)
However, Android Studio does not cache the Assets folder - there isn't any reason to cache this folder as it's expected to be raw data so the program doesn't cache it at all and expects the Assets to be modified between builds. I very highly doubt deleting the cache will solve your problem - especially when you're getting missing image errors, which are RPG Maker MV errors, not Android or Android Studio errors (Android Studio does not track the contents of the Assets folder - at all).
I doubt your device is holding a cache either as 100% of the code related to resource management comes from your RPG Maker MV project, there's nothing that would cause a device to cache code. You can rule this scenario out by uninstalling the game from your Android device then installing the APK again. I'm guessing you've already tried this.
After ruling out the fact that the zero caching is happening here, the only thing left that could be causing this RPG Maker MV error is something inside the RPG Maker MV project: RPG Maker MV errors are always due to project or Plugin bugs. These are the 99.999% of errors people get. Make sure your project is not WebGL only (test it in canvas graphics mode, the community basics Plugin can do this), make sure your project is not Windows-only (file names must be correct casing - try your project on Mac or Linux to test this) and make sure your project is not desktop-only (test it on Web). Testing on Android also proves that it is not desktop-only, and from what you're saying this test is failing.
Because this is not a WebGL exception, the blame isn't WebGL support on your device.
Process of elimination is that a Plugin is misbehaving. The fact that you refuse to tell us what Plugins you are using makes me even more convinced that this is the case (combined with the fact that 99.999% of everyone else's errors have been Plugin related so far). You need to tell us what Plugins you are using, otherwise we cannot help you. Help us help you is what I'm asking for.
Further things to diagnose: When is this error appearing? Does it appear at the title screen when the game loads? Does it appear when a map loads? Does it appear after New Game or Continue? What is the file path of the image in the error? If a regular show picture from a show picture command is causing the title screen to crash then you know for certain that it's a Plugin causing the problem (attempting to pe-load images from some kind of cache or compressed data directory). Show picture will only load a picture when it is used on the map.
The error appears when I try to copy the WWW folder to Android Studio. Crashes do not occur in RPG Maker MV anywhere. This is again, completely isolated into Android Studio and in no other deploy program that I've used.
I also cannot delete assets folder, it's grayed out. That's why i am certain this issue is related to somehing in Android Studio, but I have no idea how it's asking for a picture that wasn't even in present when I deployed the game or copied the WWW to there in the first place.