Volume panel should not clear system ui flags.

Remove private WM flag (used only by the volume panel) to force
the clearable system UI flags clear.

Instead, always lay out the volume panel as if the bars are visible,
as we do for system dialogs - and don't clear the flags.

Reverts part of I25f29af5b6518aba695b64a75977ae240b742118

Bug:11986621
Change-Id: I842729d729d2a6eb2a687d0b930ddb160c6a1b57