Typically, on Android devices, "Download" directory is in /storage/sdcard0/ or /storage/emulated /sdcard or something similar to that.
I don't know where it would be on Chromebook with Android emulator.
On native Android devices Aquamail can store configuration/account setup backup to a cloud drive (Google Drive, Dropbox, etc.), as long as the corresponding app is providing access to that. Under Android emulator on Chromebook, can you see your cloud drive using one of the Android apps, e.g. "Files" (or "Download" app.)? It would be really up to that Android emulator to provide that environment that allows apps to interact between them. And from what I read, - that emulator is still not providing all functionality of Android.