The cloud storage is in fact git under the hood. On desktop, you can also locally store in the git format: Initialize a repository and save to the directory with a file name [BRANCH], that is bracket, then the branch name and then another bracket. This is a normal git repository and you can do with it whatever you want.
On mobile, this is currently not possible: We decided that maintaining other cloud/local storages would lead to an infinite number of user requests and frustration on both sides.
What you can do on your own risk is to build the android app yourself after changing the address of the cloud server so that it points to your own server. But then you are on your own. This is a bit harder, but this is on purpose because it is not for the faint hearted.
Best
Robert