Fix Settings app crash after enabled multi-user

Bug: 8128225
Change-Id: I2918a5ec36e861a1ce4b2e67cf7607929732a7bf
Prevent cursor from causing global reference leaks.

To fix this the ContentQueryMap needs to be closed
when onStop is called so any global references can
be reclaimed.

Change-Id: I0c679c10b0d75ab0aac2fe499563c57d934e318a
avoid stopping app in case of accessing empty db

When accessing empty database, Setting App is stopped, sometimes.
To avoid stopping app, check null for cursor

Change-Id: I71ec067f502d12a9215a9abdbe9e23fc07af17bb