Guard against exception when reg/unreg content observer

When an app injects an entry to Settings with dynamic title/summary, and
disables its content provider at runtime, Settings will crash while
trying to registering/unregistering the data observer.

Fix: 337567627
Test: manual
Flag: EXEMPT bugfix
Change-Id: I9c7f689c6696d91f0b8e40113a8df10375930ede