Hide taskbar also when bottom sheet dialog is visible

This changes SysUIState to set when the SYSUI_STATE_DIALOG_SHOWING flag to  true even when a bottom sheet dialog is visible.

In a future refactor we should make SystemUIDialog manager propagate the information about SYSUI_STATE_DIALOG_SHOWING correctly, per display, instead of reporting the flag change from each SystemUIDialog subclass.

Bug: 415915352
Test: SystemUIBottomSheetDialogTest
Flag: NONE - small bugfix
Change-Id: Ie4a5ea99d0290a1c9daeaa39239a746036577c51