Fixed a Null Pointer Error in Bluetooth Settings.

+ The root of the problem is that the callback for a device change may be
set before the fragment has a layout.  Then if a change does come in,
getView() would return null.  This fix delays registering the callback
until appropiate.

Bug: 17404518
Change-Id: Ibd0e90f82593d45729f427bcc2e40fe9195eb42a