Fix login view jumping issue when keyboard appearing

[kkpan11]

Modify to adapt to auto-layout mechanism

References:

• https://developer.apple.com/library/archive/technotes/tn2154/_index.html
• https://www.hackingwithswift.com/example-code/uikit/how-to-adjust-a-uiscrollview-to-fit-the-keyboard

[kkpan11]

For the disabling btnUserAgreement on https://github.com/Ptt-official-app/Ptt-iOS/pull/70

Modify some checking logic for adjusting scrollview visible area for now https://github.com/Ptt-official-app/Ptt-iOS/pull/71/commits/008e293aa57c04fc0799de9c8f09aa81adcb8f6c