Dynamically set the identity number in identity switch

[hanwencheng]

Current Identity Switch with more than 5 identity will change the list to a scrollable view.

Identity display limit is fixed to 5. It fits well on the smallest screen like in iPhone SE, but on device with big screen, will still have a big margin vertically.

It would be better if we could programmatically set the identity display limit according to the height of the device screen.

It could be seen here.

Android Big Screen	Iphone 6	Iphone SE

Also open for other ways to styling the list.