I think what you're looking for is to increase the Gap Limit, see
How can I pre-generate new addresses?:
[wallet.create_new_address(False) for i in range(50)]
(read reference link first!)
I've never used Electrum on Android, so I'm not sure if it has a console to enter this.
Actually you have to pass True to that function for change addresses.
@OP I suggest you restore from seed on an offline PC and then do the cold storage dance:
http://docs.electrum.org/en/latest/coldstorage.htmlThis is the way to spend from cold storage in electrum.
If you insist on exporting the private key then understand that knowledge of the private key + your xpub gives an attacker complete access to your wallet. So it is not safe. But if you want to do it anyway then you will still need to restore from seed on an offline PC, run the command LoyceV gave you (with the small modification I mentioned above) until that address shows up in your wallet (
how to check). Then simply right click on it on the addresses tab (view > show addresses) and choose to view the priv key