[QSDetailedView] Implement the cast details subtitle showing logic

Hide the subtitle from the xml view.

When adapter list is not empty anymore, remove the subtitle. (set
the subtitle to "").

Bug: 378513588
Flag: com.android.systemui.qs_tile_detailed_view
Test: Existing unit tests still pass.
Change-Id: I3f8573018dbff035b0fb507cbc3d88bfcae4e5f1