Migrated SimpleTextInput to unified API

- Added NSTimer as parameter to anonymous delegate of Action<NSTimer>
(was originally Action before migration)
- Cast nfloat to float
- Cast nint to int
- Changed overriden property name from 'end' to 'End' (inherited from
type UITextRange)
- Changed overridden property name from 'start' to 'Start' (inherited
from type UITextRange)
MigratedSimpleText input to unified API

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