[TIMOB-20491] Production builds should not use builtin keystore

If the user does not specify a keystore, we will always [fall back on our builtin keystore](https://github.com/appcelerator/titanium_mobile/blob/981b9f8f61e58e0c74789e6db48ae8feb007b16a/android/cli/commands/_build.js#L1748-L1755). To enforce security best practices we should require the user to specify a keystore for production builds.