Fix nondismissing vol dialog after config change

After a config change, the volume dialog calls init again
which sets mShowing to false.  After init is called in showH(),
mShowing should be set to true to reflect the actual state of
the volume dialog.

Test: manual
Change-Id: I0afd03eb33ff2a85856dbc0882fbd9ed46ac4c99
Fixes: 117797128