[bc25] Collapse the shade when switching to bouncer.

Bug: 356596436
Bug: 369513770
Flag: com.android.systemui.scene_container
Flag: com.android.systemui.dual_shade
Test: Added unit tests.
Test: Existing unit tests still pass.
Test: Verified manually by opening both shades on lockscreen, long
 pressing on Bluetooth, and observing that the shade is closed while the
 bouncer opens.
Change-Id: I5edc7e72e65c5af729a957d0ad962b7c1fb2e5c2