SystemUI: unregister ContentObserver

We were leaking memory.

PS2: move instanciating to the SearchPanelView constructor
PS3: more possible memory leaks

Change-Id: Ie9ddf00c698692f418cb8ef473dff2b1aa05135d
Signed-off-by: Dirk Rettschlag <dirk.rettschlag@gmail.com>