ui: Refactor the declaration orders.

By grouping similar kinds together, in an order of types, constants,
ctor/dtor, all other methods and data members.

Also rename ScreenRecoveryUI::density_ to ScreenRecoveryUI::kDensity to
align with others.

Test: mmma bootable/recovery
Change-Id: I1ba2d15c05ba7be8c39762f3d9dadf1fb2130de4