[Bouncer] Add ui changes to auto confirmation.

Add pin shape avd animations to non 6 digit and 6 digit auto
confirmation screens.

Add transparent backspace key that hides and shows when text length > 0
and is auto confirmation.

Fixes: 264190150
Test: Changing pin digit size from 6 to > 6 to < 6.
Test: Inputting wrong pin code and resetting.
Test: Feature flag on/off
Change-Id: I47ce330e15e01f3a8a435eadaa1bb2d4926cbbe6