Revert "SystemUI: unregister ContentObserver"

This reverts commit 5d2d682a53e864e64e430bc649aa47e903e7e5a8.
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>