[flexiglass] Ignores PIN input past the hinted length.

If the user already put in a PIN that's the same length as the hinted
length, ignore any further digit input.

Fix: 330484299
Flag: ACONFIG com.android.systemui.scene_container DEVELOPMENT
Test: added unit tests
Test: manually verified that quickly entering numbers past the correct
hinted text stops at the max length and unlocks the device
Test: that repeating the above works
Test: that leaving the bouncer when the PIN is partially entered and
returning correctly clears out the entered PIN and correctly accepts the
correct PIN to unlock
Test: that entering the wrong PIN fails properly
Test: that entering a PIN longer than the real one when not hinted and
not autoconfirm still works as well

Change-Id: Ie6b429f906d88a63cae4c410fd2b97d247288c4c