[flexiglass] Adds support for "enhanced PIN privacy"

Fix: 308977777
Test: unit tests added for repository and view-model
Test: manully verified with the setting on and off that the digit
buttons on the PIN bouncer don't and do animate when touched,
respectively
Test: manually verified that the setting value has no bearing on whether
the non-digit buttons (delete and enter) are animated when touched: they
always animate
Flag: ACONFIG com.android.systemui.scene_container DEVELOPMENT

Change-Id: Idf116b27ccd590a3f57bf6b6c35933ae2e3075d6