Android app: Validate server URLs

[StoyanDimitrov]

Make proper URL validation of the user input in:

• default server
• new user for new server

Currently one can add invalid characters in those server URL fields.

[binwiederhier]

Fixed in binwiederhier/ntfy-android@fc27b0c

[binwiederhier]

Note: I opted for keeping a very simple URL validation (basically anything starting with http?://...), because it's too hard and not worth it to do anything else.