[flexiglass] Ignore passwords shorter than 4 chars.

Fix: 306517554
Test: unit tests added
Test: manually verified that entering more than 10 incorrect, short
passwords is ignored and throttling didn't kick off.
Flag: ACONFIG com.android.systemui.scene_container DEVELOPMENT

Change-Id: I7de5c3d5d7691555bc5acb7d9628f63e04718f9f