[flexiglass] Clear bouncer input when it's dismissed.

Fix: 310960073
Test: Manually verified by typing a password (and PIN), dismissing the
 bouncer, reopening the bouncer, and observing that the input is empty.
Test: Added new unit tests.
Test: Existing unit tests still pass.
Flag: ACONFIG com.android.systemui.scene_container DEVELOPMENT
Change-Id: I1a38e2714130533efc67d3c1b10ec1edbb4330d2