SystemUI: fix battery text % dropping to 0

When the statusbar was recreated, the battery
text view was using the old reference and callback
(which didn't exist anymore). So just recreate the
text view with the statusbar.

Change-Id: I1f246a16d0a7d84054052372b68ec220eb4a7d27
Signed-off-by: Roman Birg <romanbirg@gmail.com>