[flexiglass] Don't dim screen or go to sleep while typing a password.

Fix: 306518856
Test: Manually verified by typing a long password, waiting ~4 seconds
 after each character typed, and observing that the screen is not
 dimmed. The same verification was done for PIN bouncer as well.
Test: Added new unit tests.
Test: Existing unit tests still pass.
Flag: ACONFIG com.android.systemui.scene_container DEVELOPMENT
Change-Id: Icabae81246217741a9fc4e5b409c4c6c55f49eff