I noticed the other day that iOS correctly detects that SAML is being used and authenticates automatically. Android on the other hand requires you to add
"auth": [ { "type": "saml2.web.post" } ]
property to the logon context in the sap.Logon.init call to force SAML authentication to the correct IDP.
Please let me know if this fixes the issue for you or you require further help on this specific issue.