[flexiglass] Fix minor issues with PIN input

Clip the PIN input field so that it does not draw the digits outside of
its content area. Also center the digits horizontally.

Test: manual verification
Flag: com.android.systemui.scene_container
Bug: 431944660
Bug: 433510483
Change-Id: Idef81039262dc9cf1a4ba4948caaab1d3488419a