Hey There!
I have the very-very same problem. I've just bought aqua mail Pro in order to achieve this (O365 on my own mobile phone so I dont want Microsofts crappy policy enforcer anywhere near my personal device)
The solution which worked well for me was setting the user agent header of the webview as well as setting wide viewport (aka desktop mode)
stackoverflow.com/questions/48051857/how-to-request-desktop-in-a-webview
I've used a simple linux desktop firefox useragent and could log in successfully. However having my own fullscreen androidapp browser is not really as good as having a full-fledged email client..
The only convinience it provides is: no need for URL bar, no need to use a bookmark to get to the login page..
My feature request / Advice would be for the team -if possible- to add some sort of debug option where we can change the user agent string. If it helps I'm more than happy to paste the code piece which allowed me to log in.
Keep up the awesome work you've done so far
and hope everyone is getting better (C) Soon ^TM.
Chheers!