Stash taskbar when a SysUI dialog appears.

SYSUI_STATE_DIALOG_SHOWING is used by all implementations of SystemUIDialog, which includes QS dialogs and the power menu.
This change is a no-op for QS dialogs as the taskbar is already stashed when the notification panel is expanded.

Fix: 341613141
Test: manually with power menu dialog
Flag: EXEMPT bugfix
Change-Id: I528b30ee45f93a5df2b9d6f4d3b3c32e89935417