Add support for auto-confirmed PINs

This changes the PIN code to a List<Int>, PIN codes with leading zeros
cant be distinguished from shorter PINs.

This CL does not yet introduce the hint shapes (will requires AVDs), but
updates the visibility/appearance of the backspace/confirm button
already.

Bug: 284429000
Test: unit tests
Change-Id: I3ebfd68586f03a0d280b5c56d95444895dd05ade