Remove redundant updateDatabase method in FeatureItem
It is the same the the parent method
Moved the initialization of DB from onCreateView to onCreate
In MainActivity, cancel getItemListTask when the activity is stopped
Fix crash when adding new item while getItemListTask is in progress
This seems to be due to that getItemListTask closes the database
connection while the new activity tries to access it. The current
workaround is to take out all codes that closes sqlite database. There
might be a bit of performance loss due to this.